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/.golangci.yml

@@ -0,0 +1,55 @@
+run:
+  tests: true
+  skip-dirs:
+    - _examples
+
+output:
+  print-issued-lines: false
+
+linters:
+  enable-all: true
+  disable:
+    - maligned
+    - megacheck
+    - lll
+    - gocyclo
+    - dupl
+    - gochecknoglobals
+    - funlen
+    - godox
+    - wsl
+    - gomnd
+    - gocognit
+
+linters-settings:
+  govet:
+    check-shadowing: true
+  gocyclo:
+    min-complexity: 10
+  dupl:
+    threshold: 100
+  goconst:
+    min-len: 8
+    min-occurrences: 3
+
+issues:
+  max-per-linter: 0
+  max-same: 0
+  exclude-use-default: false
+  exclude:
+    # Captured by errcheck.
+    - '^(G104|G204):'
+    # Very commonly not checked.
+    - 'Error return value of .(.*\.Help|.*\.MarkFlagRequired|(os\.)?std(out|err)\..*|.*Close|.*Flush|os\.Remove(All)?|.*printf?|os\.(Un)?Setenv). is not checked'
+    - 'exported method (.*\.MarshalJSON|.*\.UnmarshalJSON|.*\.EntityURN|.*\.GoString|.*\.Pos) should have comment or be unexported'
+    - 'composite literal uses unkeyed fields'
+    - 'declaration of "err" shadows declaration'
+    - 'should not use dot imports'
+    - 'Potential file inclusion via variable'
+    - 'should have comment or be unexported'
+    - 'comment on exported var .* should be of the form'
+    - 'at least one file in a package should have a package comment'
+    - 'string literal contains the Unicode'
+    - 'methods on the same type should have the same receiver name'
+    - '_TokenType_name should be _TokenTypeName'
+    - '`_TokenType_map` should be `_TokenTypeMap`'