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

What would be the language to use then? We don't know the user, so we can not use user's language. We don't know the community, so we can't use community language.

Adding all texts to .yml files might be a reason to use t(..) but then translators would have to do unnecessary work.