Feature/featured matches

- Added 'featured' attribute to the Match model
- Added ability to mark general matches as featured in the admin panel
- Added ability to mark featured matches as general ones in the admin panel
- Added an API endpoint to fetch either an aggregated list of featured matches (if tournament slug is not provided) or the list of latest featured matches for the given tournament

Note:
The endpoint to retrieve aggregated list: /api/featured_matches/
The endpoint to retrieve featured matches in a specific tournament: /api/featured_matches?tournament=TOURNAMENT_SLUG
Indenting in this file and and the next is non-standard.