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/iterator.go

@@ -0,0 +1,76 @@
+package chroma
+
+import "strings"
+
+// An Iterator across tokens.
+//
+// nil will be returned at the end of the Token stream.
+//
+// If an error occurs within an Iterator, it may propagate this in a panic. Formatters should recover.
+type Iterator func() Token
+
+// Tokens consumes all tokens from the iterator and returns them as a slice.
+func (i Iterator) Tokens() []Token {
+	var out []Token
+	for t := i(); t != EOF; t = i() {
+		out = append(out, t)
+	}
+	return out
+}
+
+// Concaterator concatenates tokens from a series of iterators.
+func Concaterator(iterators ...Iterator) Iterator {
+	return func() Token {
+		for len(iterators) > 0 {
+			t := iterators[0]()
+			if t != EOF {
+				return t
+			}
+			iterators = iterators[1:]
+		}
+		return EOF
+	}
+}
+
+// Literator converts a sequence of literal Tokens into an Iterator.
+func Literator(tokens ...Token) Iterator {
+	return func() Token {
+		if len(tokens) == 0 {
+			return EOF
+		}
+		token := tokens[0]
+		tokens = tokens[1:]
+		return token
+	}
+}
+
+// SplitTokensIntoLines splits tokens containing newlines in two.
+func SplitTokensIntoLines(tokens []Token) (out [][]Token) {
+	var line []Token // nolint: prealloc
+	for _, token := range tokens {
+		for strings.Contains(token.Value, "\n") {
+			parts := strings.SplitAfterN(token.Value, "\n", 2)
+			// Token becomes the tail.
+			token.Value = parts[1]
+
+			// Append the head to the line and flush the line.
+			clone := token.Clone()
+			clone.Value = parts[0]
+			line = append(line, clone)
+			out = append(out, line)
+			line = nil
+		}
+		line = append(line, token)
+	}
+	if len(line) > 0 {
+		out = append(out, line)
+	}
+	// Strip empty trailing token line.
+	if len(out) > 0 {
+		last := out[len(out)-1]
+		if len(last) == 1 && last[0].Value == "" {
+			out = out[:len(out)-1]
+		}
+	}
+	return
+}