Task Collection Rank: Improve validity and reverse numbers

[MrSerth]

Currently, the rank attribute of task_collections is stored as a number, with the highest rank being on top of the list. With the current implementation, I see two possibilities for improvement:

1. The rank value 0 might occur multiple times, which will lead to the respective task being listed at the bottom. That behavior per-se is okay, but I would prefer to always have a valid numbering scheme stored in the database.
   
2. We could think of switching the rank to something like position and reverse the numbers (with 0 being at the top of the list).