Enforce the minimum harvest-interval

[hanswesterbeek]

The minimum interval should be 15 minutes.

• Adapt the GUI to that effect.
• Automatically migrate existing harvest-schedules at startup.

[hanswesterbeek]

I'm starting to believe that we have to fall back to a maximum of once an hour. Implementing a GUI that is clear to the user (decent validation, feedback) is too time-consuming for us right now.

Come to think of it: maybe we should simplify matters greatly by setting all schedules to once an hour. If nothing changes, nothing changes.

We need to cut down on all the random features in this thing to ever get a hold of it.

@kiivihal are you reading this? :)