Overall code review, refactoring

[maizy]

• [x] code review
• [ ] refactor all fetcher/client part
  • [ ] break client into resources classes
  • [x] repos
  • [ ] add tests with new http stubs - https://www.playframework.com/documentation/2.4.x/ScalaTestingWebServiceClients
  • [x] caching as resource extender
  • [ ] fix POST /github/reload (force reload of github resources)
  • [ ] parallel requests