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.

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -997,7 +997,8 @@ Default templates for project boards:
 
 ## i18n (`i18n`)
 
-- `LANGS`: **en-US,zh-CN,zh-HK,zh-TW,de-DE,fr-FR,nl-NL,lv-LV,ru-RU,ja-JP,es-ES,pt-BR,pt-PT,pl-PL,bg-BG,it-IT,fi-FI,tr-TR,cs-CZ,sr-SP,sv-SE,ko-KR,el-GR,fa-IR,hu-HU,id-ID,ml-IN**: List of locales shown in language selector
+- `LANGS`: **en-US,zh-CN,zh-HK,zh-TW,de-DE,fr-FR,nl-NL,lv-LV,ru-RU,ja-JP,es-ES,pt-BR,pt-PT,pl-PL,bg-BG,it-IT,fi-FI,tr-TR,cs-CZ,sr-SP,sv-SE,ko-KR,el-GR,fa-IR,hu-HU,id-ID,ml-IN**:
+     List of locales shown in language selector. The first locale will be used as the default if user browser's language doesn't match any locale in the list.
 - `NAMES`: **English,简体中文,繁體中文（香港）,繁體中文（台灣）,Deutsch,français,Nederlands,latviešu,русский,日本語,español,português do Brasil,Português de Portugal,polski,български,italiano,suomi,Türkçe,čeština,српски,svenska,한국어,ελληνικά,فارسی,magyar nyelv,bahasa Indonesia,മലയാളം**: Visible names corresponding to the locales
 
 ## U2F (`U2F`) **DEPRECATED**

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -299,6 +299,8 @@ LANGS = en-US,foo-BAR
 NAMES = English,FooBar
 ```
 
+The first locale will be used as the default if user browser's language doesn't match any locale in the list.
+
 Locales may change between versions, so keeping track of your customized locales is highly encouraged.
 
 ### Readmes