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/api.go

@@ -0,0 +1,37 @@
+package styles
+
+import (
+	"sort"
+
+	"github.com/alecthomas/chroma"
+)
+
+// Registry of Styles.
+var Registry = map[string]*chroma.Style{}
+
+// Fallback style. Reassign to change the default fallback style.
+var Fallback = SwapOff
+
+// Register a chroma.Style.
+func Register(style *chroma.Style) *chroma.Style {
+	Registry[style.Name] = style
+	return style
+}
+
+// Names of all available styles.
+func Names() []string {
+	out := []string{}
+	for name := range Registry {
+		out = append(out, name)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// Get named style, or Fallback.
+func Get(name string) *chroma.Style {
+	if style, ok := Registry[name]; ok {
+		return style
+	}
+	return Fallback
+}