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/styles/vs.go

@@ -0,0 +1,23 @@
+package styles
+
+import (
+	"github.com/alecthomas/chroma"
+)
+
+// VisualStudio style.
+var VisualStudio = Register(chroma.MustNewStyle("vs", chroma.StyleEntries{
+	chroma.Comment:           "#008000",
+	chroma.CommentPreproc:    "#0000ff",
+	chroma.Keyword:           "#0000ff",
+	chroma.OperatorWord:      "#0000ff",
+	chroma.KeywordType:       "#2b91af",
+	chroma.NameClass:         "#2b91af",
+	chroma.LiteralString:     "#a31515",
+	chroma.GenericHeading:    "bold",
+	chroma.GenericSubheading: "bold",
+	chroma.GenericEmph:       "italic",
+	chroma.GenericStrong:     "bold",
+	chroma.GenericPrompt:     "bold",
+	chroma.Error:             "border:#FF0000",
+	chroma.Background:        " bg:#ffffff",
+}))