#535 flag cards

[Mathias Kussinger]

Implement a flag feature for the cards to mark cards that require a follow up activity.
This feature requires:
- a flag flag for the cards in the database => db upgrade
- a flag toggle button on the test and drill screens
- a flag toggle button on the card edit screen(s)
- a search for flaged cards button on the category list
- a list view for cards searched by feature, here the flag flag

[Christa Runge]

Ticket moved from /p/karatasi/features/63/

Can't be converted:

• _milestone: iphone_1.8