Use Webpacker for all assets

[pixeldesu]

This pretty much moves all SCSS into Webpacker so that it doesn't require asset gems and alike anymore.

Also:

• removed nprogress as it was unused
• moved font-awesome from an asset gem to an npm package
• included Lato font locally, not from Google Fonts (Fixes Serve webfont ourselves #95)
• moved the CSS imports in JS to SCSS
• removed all the (now) unused asset gems

[codecov]

Codecov Report

Merging #295 (562abbf) into master (29923fa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #295   +/-   ##
=======================================
  Coverage   70.74%   70.74%           
=======================================
  Files         104      104           
  Lines        2287     2287           
=======================================
  Hits         1618     1618           
  Misses        669      669           

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[raccube]