Remove legacy unmaintained packages, refactor to support change default locale (#19308)

Remove two unmaintained vendor packages `i18n` and `paginater`. Changes:
* Rewrite `i18n` package with a more clear fallback mechanism. Fix an unstable `Tr` behavior, add more tests.
* Refactor the legacy `Paginater` to `Paginator`, test cases are kept unchanged.

Trivial enhancement (no breaking for end users):
* Use the first locale in LANGS setting option as the default, add a log to prevent from surprising users.

modules/paginator/paginator.go

@@ -0,0 +1,203 @@
+// Copyright 2022 The Gitea Authors.
+// Copyright 2015 Unknwon. Licensed under the Apache License, Version 2.0
+
+package paginator
+
+/*
+In template:
+
+```html
+{{if not .Page.IsFirst}}[First](1){{end}}
+{{if .Page.HasPrevious}}[Previous]({{.Page.Previous}}){{end}}
+
+{{range .Page.Pages}}
+	{{if eq .Num -1}}
+	...
+	{{else}}
+	{{.Num}}{{if .IsCurrent}}(current){{end}}
+	{{end}}
+{{end}}
+
+{{if .Page.HasNext}}[Next]({{.Page.Next}}){{end}}
+{{if not .Page.IsLast}}[Last]({{.Page.TotalPages}}){{end}}
+```
+
+Output:
+
+```
+[First](1) [Previous](2) ... 2 3(current) 4 ... [Next](4) [Last](5)
+```
+*/
+
+// Paginator represents a set of results of pagination calculations.
+type Paginator struct {
+	total     int // total rows count
+	pagingNum int // how many rows in one page
+	current   int // current page number
+	numPages  int // how many pages to show on the UI
+}
+
+// New initialize a new pagination calculation and returns a Paginator as result.
+func New(total, pagingNum, current, numPages int) *Paginator {
+	if pagingNum <= 0 {
+		pagingNum = 1
+	}
+	if current <= 0 {
+		current = 1
+	}
+	p := &Paginator{total, pagingNum, current, numPages}
+	if p.current > p.TotalPages() {
+		p.current = p.TotalPages()
+	}
+	return p
+}
+
+// IsFirst returns true if current page is the first page.
+func (p *Paginator) IsFirst() bool {
+	return p.current == 1
+}
+
+// HasPrevious returns true if there is a previous page relative to current page.
+func (p *Paginator) HasPrevious() bool {
+	return p.current > 1
+}
+
+func (p *Paginator) Previous() int {
+	if !p.HasPrevious() {
+		return p.current
+	}
+	return p.current - 1
+}
+
+// HasNext returns true if there is a next page relative to current page.
+func (p *Paginator) HasNext() bool {
+	return p.total > p.current*p.pagingNum
+}
+
+func (p *Paginator) Next() int {
+	if !p.HasNext() {
+		return p.current
+	}
+	return p.current + 1
+}
+
+// IsLast returns true if current page is the last page.
+func (p *Paginator) IsLast() bool {
+	if p.total == 0 {
+		return true
+	}
+	return p.total > (p.current-1)*p.pagingNum && !p.HasNext()
+}
+
+// Total returns number of total rows.
+func (p *Paginator) Total() int {
+	return p.total
+}
+
+// TotalPages returns number of total pages.
+func (p *Paginator) TotalPages() int {
+	if p.total == 0 {
+		return 1
+	}
+	return (p.total + p.pagingNum - 1) / p.pagingNum
+}
+
+// Current returns current page number.
+func (p *Paginator) Current() int {
+	return p.current
+}
+
+// PagingNum returns number of page size.
+func (p *Paginator) PagingNum() int {
+	return p.pagingNum
+}
+
+// Page presents a page in the paginator.
+type Page struct {
+	num       int
+	isCurrent bool
+}
+
+func (p *Page) Num() int {
+	return p.num
+}
+
+func (p *Page) IsCurrent() bool {
+	return p.isCurrent
+}
+
+func getMiddleIdx(numPages int) int {
+	return (numPages + 1) / 2
+}
+
+// Pages returns a list of nearby page numbers relative to current page.
+// If value is -1 means "..." that more pages are not showing.
+func (p *Paginator) Pages() []*Page {
+	if p.numPages == 0 {
+		return []*Page{}
+	} else if p.numPages == 1 && p.TotalPages() == 1 {
+		// Only show current page.
+		return []*Page{{1, true}}
+	}
+
+	// Total page number is less or equal.
+	if p.TotalPages() <= p.numPages {
+		pages := make([]*Page, p.TotalPages())
+		for i := range pages {
+			pages[i] = &Page{i + 1, i+1 == p.current}
+		}
+		return pages
+	}
+
+	numPages := p.numPages
+	offsetIdx := 0
+	hasMoreNext := false
+
+	// Check more previous and next pages.
+	previousNum := getMiddleIdx(p.numPages) - 1
+	if previousNum > p.current-1 {
+		previousNum -= previousNum - (p.current - 1)
+	}
+	nextNum := p.numPages - previousNum - 1
+	if p.current+nextNum > p.TotalPages() {
+		delta := nextNum - (p.TotalPages() - p.current)
+		nextNum -= delta
+		previousNum += delta
+	}
+
+	offsetVal := p.current - previousNum
+	if offsetVal > 1 {
+		numPages++
+		offsetIdx = 1
+	}
+
+	if p.current+nextNum < p.TotalPages() {
+		numPages++
+		hasMoreNext = true
+	}
+
+	pages := make([]*Page, numPages)
+
+	// There are more previous pages.
+	if offsetIdx == 1 {
+		pages[0] = &Page{-1, false}
+	}
+	// There are more next pages.
+	if hasMoreNext {
+		pages[len(pages)-1] = &Page{-1, false}
+	}
+
+	// Check previous pages.
+	for i := 0; i < previousNum; i++ {
+		pages[offsetIdx+i] = &Page{i + offsetVal, false}
+	}
+
+	pages[offsetIdx+previousNum] = &Page{p.current, true}
+
+	// Check next pages.
+	for i := 1; i <= nextNum; i++ {
+		pages[offsetIdx+previousNum+i] = &Page{p.current + i, false}
+	}
+
+	return pages
+}