Extend Bookmarklists and implement stored searches

Separated from #115 (closed)

1. A user can define several watchlists.
   • Each watchlist has a name.
   • A user should be able to put exercises onto a watchlist. Also he can remove exercises from the watchlist.
   • A watchlist can be deleted
   • A user can set an alarm, if an exercise on his watchlist was updated. The alarm is executed nightly.

Second Part for Saved Searches was transferred to #155 (closed)