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/dlclark/regexp2/syntax/fuzz.go

@@ -0,0 +1,20 @@
+// +build gofuzz
+
+package syntax
+
+// Fuzz is the input point for go-fuzz
+func Fuzz(data []byte) int {
+	sdata := string(data)
+	tree, err := Parse(sdata, RegexOptions(0))
+	if err != nil {
+		return 0
+	}
+
+	// translate it to code
+	_, err = Write(tree)
+	if err != nil {
+		panic(err)
+	}
+
+	return 1
+}