It shouldn't be possible to change the hackathon submission date after publishing

### User Story
As a hackathon applicant, I want to prevent hackathon sponsors/owners from postponing the submission date/time of a hackathon.

### Why Is this Needed
It is not honest with the hackathon participants if the submission date is postponed shortly before the original date is reached.

### Description
*Type*: Feature

### Current Behavior
A hackathon sponsor/owner can postpone the submission date/time at any time.

### Expected Behavior
A hackathon sponsor/owner can't postpone the submission date/time. Alternatively, a change of submission date *must* appear as an activity on the hackathon page. For more transparency and fairness towards the participants.

### Definition of Done
Once a hackathon has been published, the submission date cannot be changed.