Implement news preferences in a generic user preferences system
Create minimal config page
Add model News::UserConfig

* belongs to user
* stores user's preferences related to news
Render the config panel on the config page
Implement in-place toggling of receiving news digests by email
Extract mailing news digest logic from mailer to own use case
Rename dir 'spec/contexts' to 'spec/use_cases'
Rename wrongly named spec file
Remove validating presence of user_id in News::UserConfig
News digest emails: add opt-out + keep track of last sent mail
Add support for cases when the news digest should not be mailed

* Does not send it if user opted-out
* Does not send it if there are no news to display
Enable support for sending digests to all users in rake task
Subscribe users by default to digests until they opt-out
Use correct syntax for activerecord first_or_create
Make news fetcher 'interesting' include sub-themed news on 2 levels
Define 'descendants' method for a Tag
Make news fetcher 'interesting' include sub-themed news on all levels
Fix performance issue of news fetcher multi-level filter 'interesting'
Fix missing time limit in filter 'interesting'
Make ReceivingADigestEmail write results into stdout

* Allows to see what is happening with the mailing
  of digests in realtime