Frontend refactor

[s0meone]

Frontend updated to modern standards.

Including:

• ES6 React classes;
• React Router v4 instead of page.js;
• dependencies as npm modules;
• upgrade dependencies;
• include libsass, instead of Ruby sass;
• use Webpack as the main build tool, instead of Grunt;
• modular design instead of global objects;
• removed controller concept, logic and routes is managed inside React components;
• use fetch api instead of jQuery AJAX;
• removed Ruby dependency;
• add eslint code checking and fixed all current errors;
• eslint also enforces code style.

PR will be squashed before merge.

[codecov-io]

Current coverage is 90.20% (diff: 100%)

Merging #7 into master will not change coverage

@@             master         #7   diff @@
==========================================
  Files            70         70          
  Lines          1399       1399          
  Methods           0          0          
  Messages          0          0          
  Branches        140        140          
==========================================
  Hits           1262       1262          
  Misses           80         80          
  Partials         57         57          

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