A lot of the code dealing with collections assumes that the editing is
done by the owner (true assumption, for now), and therefore the encryption
key is derived from the master key (not true anymore, as it could be a
stored version of the old key). This commit removes this wrong assumption.
This adds support for tasks via OpenTasks.
https://github.com/dmfs/opentasks
Need the OpenTasks client for it to be used.
Currently you can't create new task lists. You can only have the default
one, but that's just a UI thing.
Fixes#7