Server-side syntax highlighting for all code (#12047)

* Server-side syntax hilighting for all code

This PR does a few things:

* Remove all traces of highlight.js
* Use chroma library to provide fast syntax hilighting directly on the server
* Provide syntax hilighting for diffs
* Re-style both unified and split diffs views
* Add custom syntax hilighting styling for both regular and arc-green

Fixes #7729
Fixes #10157
Fixes #11825
Fixes #7728
Fixes #3872
Fixes #3682

And perhaps gets closer to #9553

* fix line marker

* fix repo search

* Fix single line select

* properly load settings

* npm uninstall highlight.js

* review suggestion

* code review

* forgot to call function

* fix test

* Apply suggestions from code review

suggestions from @silverwind thanks

Co-authored-by: silverwind <me@silverwind.io>

* code review

* copy/paste error

* Use const for highlight size limit

* Update web_src/less/_repository.less

Co-authored-by: Lauris BH <lauris@nix.lv>

* update size limit to 1MB and other styling tweaks

* fix highlighting for certain diff sections

* fix test

* add worker back as suggested

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/alecthomas/chroma/lexers/t/termcap.go

@@ -0,0 +1,42 @@
+package t
+
+import (
+	. "github.com/alecthomas/chroma" // nolint
+	"github.com/alecthomas/chroma/lexers/internal"
+)
+
+// Termcap lexer.
+var Termcap = internal.Register(MustNewLexer(
+	&Config{
+		Name:      "Termcap",
+		Aliases:   []string{"termcap"},
+		Filenames: []string{"termcap", "termcap.src"},
+		MimeTypes: []string{},
+	},
+	Rules{
+		"root": {
+			{`^#.*$`, Comment, nil},
+			{`^[^\s#:|]+`, NameTag, Push("names")},
+		},
+		"names": {
+			{`\n`, Text, Pop(1)},
+			{`:`, Punctuation, Push("defs")},
+			{`\|`, Punctuation, nil},
+			{`[^:|]+`, NameAttribute, nil},
+		},
+		"defs": {
+			{`\\\n[ \t]*`, Text, nil},
+			{`\n[ \t]*`, Text, Pop(2)},
+			{`(#)([0-9]+)`, ByGroups(Operator, LiteralNumber), nil},
+			{`=`, Operator, Push("data")},
+			{`:`, Punctuation, nil},
+			{`[^\s:=#]+`, NameClass, nil},
+		},
+		"data": {
+			{`\\072`, Literal, nil},
+			{`:`, Punctuation, Pop(1)},
+			{`[^:\\]+`, Literal, nil},
+			{`.`, Literal, nil},
+		},
+	},
+))