add css extraction and minification to webpack

[silverwind]

This changes the CSS output of webpack to output to the public/css directory instead of inling CSS in JS. This enables CSS minification and autoprefixer based on browserslist which would otherwise not be possible.

The result of this change is two new output files currently:

• public/css/swagger.css
• public/css/gitgraph.css

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c43dec4). Click here to learn what that means.
The diff coverage is 51.69%.

@@            Coverage Diff            @@
##             master    #9944   +/-   ##
=========================================
  Coverage          ?   42.24%           
=========================================
  Files             ?      610           
  Lines             ?    80295           
  Branches          ?        0           
=========================================
  Hits              ?    33919           
  Misses            ?    42214           
  Partials          ?     4162

Impacted Files	Coverage Δ
models/models.go	61.71% <ø> (ø)
models/repo_list.go	76.81% <ø> (ø)
models/issue.go	54.81% <ø> (ø)
routers/api/v1/org/hook.go	0% <0%> (ø)
cmd/admin.go	0% <0%> (ø)
routers/org/teams.go	0% <0%> (ø)
models/pull_sign.go	34.86% <0%> (ø)
routers/api/v1/repo/issue_subscription.go	0% <0%> (ø)
routers/api/v1/repo/release.go	46.2% <0%> (ø)
routers/api/v1/repo/key.go	20.37% <0%> (ø)
... and 88 more

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[silverwind]

In case anyone's wondering: webpack build time is pretty much unchanged.

[silverwind]

Rebased and added some trailing commas in webpack.config.js for consistency.