move highlight.js to npm/webpack

[silverwind]

• introduced window.config to help with js-based lazy-loading
• adjusted webpack chunk naming to avoid 'vendors~name.js' that webpack defaults to for vendor chunks.
• added theme class to html and prefixed all selectors. this is neccesary so that the theme styles win over the lazy-loaded ones.

[silverwind]

There are regressions in the dark theme rendering, will figure something out (likely making its hljs selectors !important).

Edit: Fixed those by adding the parent selector to only those .hljs styles. I'm sure there are cleaner approaches to themeing, but this should do for now.

[silverwind]

Rebased and did a few improvments. Good to go from my side.

[codecov-io]

Codecov Report

Merging #10011 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #10011      +/-   ##
==========================================
- Coverage   42.27%   42.26%   -0.01%     
==========================================
  Files         611      611              
  Lines       80407    80407              
==========================================
- Hits        33989    33987       -2     
- Misses      42239    42241       +2     
  Partials     4179     4179

Impacted Files	Coverage Δ
routers/api/v1/admin/repo.go	0% <0%> (ø)	⬆️
models/repo.go	49.66% <0%> (-0.14%)	⬇️
services/pull/patch.go	67.92% <0%> (ø)	⬆️

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[silverwind]

highlight.js/styles/github.css moved to main CSS bundle, this resolves the loading order issue with the dark theme so it works without modification while increasing the main CSS bundle by around 1kB.