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/yuin/goldmark-highlighting/README.md

@@ -0,0 +1,66 @@
+goldmark-highlighting
+=========================
+
+goldmark-highlighting is an extension for the [goldmark](http://github.com/yuin/goldmark) 
+that adds syntax-highlighting to the fenced code blocks.
+
+goldmark-highlighting uses [chroma](https://github.com/alecthomas/chroma) as a
+syntax highlighter.
+
+Installation
+--------------------
+
+```
+go get github.com/yuin/goldmark-highlighting
+```
+
+Usage
+--------------------
+
+```go
+import (
+    "bytes"
+    "fmt"
+    "github.com/alecthomas/chroma/formatters/html"
+    "github.com/yuin/goldmark"
+    "github.com/yuin/goldmark/extension"
+    "github.com/yuin/goldmark/parser"
+    "github.com/yuin/goldmark-highlighting"
+
+)
+
+func main() {
+    markdown := goldmark.New(
+        goldmark.WithExtensions(
+            highlighting.Highlighting,
+        ),
+    )
+    var buf bytes.Buffer
+    if err := markdown.Convert([]byte(source), &buf); err != nil {
+        panic(err)
+    }
+    fmt.Print(title)
+}
+```
+
+
+```go
+    markdown := goldmark.New(
+        goldmark.WithExtensions(
+            highlighting.NewHighlighting(
+               highlighting.WithStyle("monokai"),
+               highlighting.WithFormatOptions(
+                   html.WithLineNumbers(),
+               ),
+            ),
+        ),
+    )
+```
+
+License
+--------------------
+MIT
+
+Author
+--------------------
+Yusuke Inuzuka