Allow commitments to be deleted

[AndreiBarsan]

This is a minor one, but while setting up the app and getting used to it, I accidentally added a commitment for a habit, but was unsure how to undo it, even though I was not actually supposed to do that that day (i.e., I committed to exercise that day, but it was a rest day anyway).

It would be nice to be able to undo set commitments, at least for a limited time, e.g., 15 minutes or even less, after they are made.