Update to Rails 6, various fixes to make it work

[adnikiforov]

So I thought it will be a good idea to update to Rails 6. Obviously, there is many break changes, but I did not changed anything aside from updating and repairing things. Also didn't touched specs, they should be reworked from scratch (and they are broken anyway)

What I did:

• Updated to latest Rails 6 release
• Removed cancan in favor of cancancan (cancan is deprecated)
• Removed div_for as deprecated since Rails 5
• Added Rails 6 assets manifest
• before_filter -> before_action
• A little bit of code formatting using rubocop
• Regenerated binstubs

Further recommendations:

• We desperately need to get rid of ERB and use slim instead. Damn ERB is super ugly.
• Specs should be redone from scratch.
• Get rid of css3buttons.

DO NOT MERGE at the moment. We need to release old stable release with Rails 4 first, as a starting point.

[adnikiforov]

Wait for #75