Move generic functions away from PollViewForm

Problem/Motivation

We have a few very generic functions in the PollViewForm class. I think it makes sense if we would move those to the Poll entity class or into a helper class so other modules/developers van profit and call those functions as well.

Proposed resolution

- isCancelAllowed https://git.drupalcode.org/project/poll/-/blob/2.0.x/src/Form/PollViewFo...
- isVotingAllowed https://git.drupalcode.org/project/poll/-/blob/2.0.x/src/Form/PollViewFo...

Remaining tasks

MR
Update release note as this is a potentially breaking change

User interface changes

N/A

API changes

New public methods available

Data model changes

N/A