Add support for visual regression tests

[reekitconcept]

• add cypress plugin dependency

• add cypress configuration files for visual targets

• configure Makefile

• configure github plugin for CI

• customize and fix image snapshot generation

  • customize snapshots folder location
  • fix scrollability of css for full-page screenshots
  • fix lazy image loading
  • disable hover effect on images
  • disable hyphenation (currently disabled but code is available)
  • limit browser to firefox only

• test utilities

  • generate series of tests for different resolutions
  • generate series of tests visiting a list of urls
  • cumulative testing

• add base.js test that

  • tests two pages in anonymous view and serves as an example
  • test a list of pages as an example
  • same in authenticated mode

[sneridagh]

@reekitconcept Looks awesome! LGTM, however, I'm missing some docs, afterwards, let's merge it right away.

• how to add new tests/snapshots
• how (if we can) run locally, etc...

@tisto We need to talk to you about how to deal with that last ones.