Initial poll implementation

[BentiGorlich]

• add the ability to add a poll to Entry, EntryComment, Post and PostComment
• Polls:
  • can be multiple choice
  • can contain an unlimited number of choices
  • can expire
• Add a hidden form for the poll to all create forms, the form appears when a checkbox is checked
• The ActivityPub implementation is the same as on Mastodon: https://docs.joinmastodon.org/spec/activitypub/#Question
• Add tests for the ActivityPub implementation and the Api controller
• The api had to be shifted a bit -> the serialize... functions in the base api take only the entity classes now instead of their dto variant -> touched a lot of api classes because of this

I will add images when this moves out of the draft phase :)

Closes #647

[BentiGorlich]

Open things:

• This does not yet send an Update activity upon a new vote, which I though Mastodon also doesn't do, but there is an fep for this that was also mentioned in the issue and it says that an implementation should do it: https://codeberg.org/silverpill/feps/src/branch/main/9967/fep-9967.md
• it does not yet use the closed property, which it should do if endTime is not provided
• the Poll AP object does not yet have an updated field.
• All in all some AP stuff has to be done
• one cannot yet manually refresh the results
• one cannot yet jump to the results without voting