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/mattn/go-isatty/.travis.yml

@@ -1,13 +1,14 @@
 language: go
+sudo: false
 go:
+  - 1.13.x
   - tip
 
-os:
-  - linux
-  - osx
-
 before_install:
-  - go get github.com/mattn/goveralls
-  - go get golang.org/x/tools/cmd/cover
+  - go get -t -v ./...
+
 script:
-  - $HOME/gopath/bin/goveralls -repotoken 3gHdORO5k5ziZcWMBxnd9LrMZaJs8m9x5
+  - ./go.test.sh
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)