Upgrade to Rails 5.0

[ysbaddaden]

Begins the upgrade from Rails 4.2 to Rails 5.0

• [x] Upgrade to Rails 5.0
• [x] Fix deprecations (alias_method_chain, ...)
• [x] Vendor jquery & jquery-ui to avoid frontend upgrade issues
• [x] Upgrade to DecentExposure 3 needs changes in create/update with strong parameters
• [x] Drop includes-count gem (abandoned, used once, breaks ActiveRecord)
• [x] Drop active_model_serializers gem (used once along with includes-count)
• [x] Upgrade Devise (gem is updated, seems to work)
• [ ] Fix failures catched by the unit test suite (one last failing spec)
• [x] Fix failures catched by the integration test suite
• [ ] Enable Rails 5.0 default settings