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/remap.go

@@ -0,0 +1,80 @@
+package chroma
+
+type remappingLexer struct {
+	lexer  Lexer
+	mapper func(Token) []Token
+}
+
+// RemappingLexer remaps a token to a set of, potentially empty, tokens.
+func RemappingLexer(lexer Lexer, mapper func(Token) []Token) Lexer {
+	return &remappingLexer{lexer, mapper}
+}
+
+func (r *remappingLexer) Config() *Config {
+	return r.lexer.Config()
+}
+
+func (r *remappingLexer) Tokenise(options *TokeniseOptions, text string) (Iterator, error) {
+	it, err := r.lexer.Tokenise(options, text)
+	if err != nil {
+		return nil, err
+	}
+	var buffer []Token
+	return func() Token {
+		for {
+			if len(buffer) > 0 {
+				t := buffer[0]
+				buffer = buffer[1:]
+				return t
+			}
+			t := it()
+			if t == EOF {
+				return t
+			}
+			buffer = r.mapper(t)
+		}
+	}, nil
+}
+
+// TypeMapping defines type maps for the TypeRemappingLexer.
+type TypeMapping []struct {
+	From, To TokenType
+	Words    []string
+}
+
+// TypeRemappingLexer remaps types of tokens coming from a parent Lexer.
+//
+// eg. Map "defvaralias" tokens of type NameVariable to NameFunction:
+//
+// 		mapping := TypeMapping{
+// 			{NameVariable, NameFunction, []string{"defvaralias"},
+// 		}
+// 		lexer = TypeRemappingLexer(lexer, mapping)
+func TypeRemappingLexer(lexer Lexer, mapping TypeMapping) Lexer {
+	// Lookup table for fast remapping.
+	lut := map[TokenType]map[string]TokenType{}
+	for _, rt := range mapping {
+		km, ok := lut[rt.From]
+		if !ok {
+			km = map[string]TokenType{}
+			lut[rt.From] = km
+		}
+		if len(rt.Words) == 0 {
+			km[""] = rt.To
+		} else {
+			for _, k := range rt.Words {
+				km[k] = rt.To
+			}
+		}
+	}
+	return RemappingLexer(lexer, func(t Token) []Token {
+		if k, ok := lut[t.Type]; ok {
+			if tt, ok := k[t.Value]; ok {
+				t.Type = tt
+			} else if tt, ok := k[""]; ok {
+				t.Type = tt
+			}
+		}
+		return []Token{t}
+	})
+}