New vote delegation option

[Elblinator]

Add an option to delegate the vote to multiple other users. Only one of them can vote for this delegation, the other will be rejected afterwards. This requires switching the relation from 1:n to n:m with a migration. Additionally, a new setting is required to control to how many people one can delegate his vote, which should default to 1.

[ostcar]

The current behavior is the multi mode.

You first have to add the option to the backend. Afterwards I can take it into account in the vote service.

[jsangmeister]

The current behaviour is a mix of the two: you can only delegate your vote to (at most) one person, and at least in the client a user cannot vote if he delegated his vote. The vote service does not seem to have this restriction. This should be fixed regardless of this issue in one way or another - probably the client way, since that is what users currently actually experience.

[jsangmeister]

extracted the bug part into OpenSlides/openslides-vote-service#225

[jsangmeister]

I updated the issue description as discussed. The distinction whether or not a user can vote for themselves after the delegation is split up into #2370.