Single community filter

- [x] Remove `single_community` filter
- [x] Add `community_not_found_redirect` configuration option
- [x] Add new 404 error page 'Community not found'

How to find current community:

1. Try to find community by domain
1. If there is only one community, use it
1. Otherwise, community can not be found

What to do if community is not found

1. If `community_not_found_redirect` option is set, redirect to that URL
1. Otherwise, show 'Community not found' error page

We should log (+ airbrake?) the case where count > 1. This is a configuration error that should not happen.