Email confirmation required by default, Devise's confirmable removed

**Features:**
Email confirmation is now required for all communities

- [x] Remove optional email confirmation related code
- [x] Remove `email_confirmation` column from `community`
- [x] Remove Devise's `confirmable` related code
- [x] Remove confirmable related columns from `people`

**Todo**
This is again a bit difficult change which affects multiple places of the application, so this needs some extra testing effort:

- [x] How to skip confirmation development mode?
- [x] Test joining a community
- [x] Test FB joining

**Manual migrations**
- [ ] There are few emails that needs to be confirmed by hand.