Allow deleting a poll

Part of #975

What does this MR do?

Adds a button that allows the author of a poll to delete it. As for the "edit poll" function, this is only possible within the first hour.

This is necessary because sometimes people create the same poll multiple times and want to delete it again. Since this only allows deletion before the start time of a poll, I implemented it so that the poll is completely removed instead of setting it to cancelled.

How confident are you it won't break things if deployed?

Quite sure, I can't think of anything that could go wrong

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as uservoting1
3. Go to Göttingen -> Abstimmungen and create a new poll
4. After being redirected to the poll, click "Abstimmungen abbrechen" and confirm
5. Go to Göttingen -> Abstimmungen: the poll should not be in the list anymore

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

Angelegte Abstimmungen können innerhalb der ersten Stunde wieder gelöscht werden.