Revert increase limit

With this modification I've added the following improvements:

*Removed a statement that was doing nothing (please check if it's so)
*Improved usability of the increase new/review card daily limit by showing to the user the number of cards that are left in the deck excluding the ones that are currently under review. This number is equal to the maximum number the user can input in the spinbox. The maximum value allowed in the spin box has been updated accordingly. 

Example:
Consider a new deck. There are 100 (new) cards in the decks. The user new card limit is 10. He has learned 5 new cards. 
In the increase new card limit dialog now the number shown is 90 (before was 95).

*Add two buttons to reset the new/review card limit extension
Sometimes happens to me that I extend the new card limit for today, but than I change my mind. At this point I can't proceed reviewing the cards because also the new ones (that I don't want to learn) will be shown.
Another use case is that I extend a lot the review card limit, but then I change my mind and I would like to finish only the cards in 'Learning' state (the red ones) but new green cards will be shown making me loose time.

To work, the improvement of the two buttons needs two extra field in the deck schema. I've added them in a bad place (inside scheduler.py) but I guess they should added in storage.py. Before merging the pull request I ask you to discard that commit (cherry pick the others) and add the proper code to update the deck schema (I'm not sure how to do it).