New setting for restricting delegator rights part 2

[Elblinator]

Make backend changes for OpenSlides/openslides-client#3518

We need one new setting for the proposed functionality, I propose the following names and functions:

• users_forbid_delegator_to_vote: Users who have delegated their votes in this meeting may not vote for themselves

This will be off by default to preserve current functionality, so there's no migration necessary.

Predictably this will require changes in poll.vote, so please don't forget to update the documentation

[jsangmeister]

Predictably this will require changes in poll.vote, so please don't forget to update the documentation

Moreover, this will require changes in the vote service (@ostcar)

[ostcar]

When I remember correctly, the feature was slightly different. When this setting is set, then at all time, there can be only one user, that can vote for a user. This means, that when the setting is set, then the vote can only be delegated to one user. When the setting is not set, then the vote can be delegated to many users.

[jsangmeister]

Related issues:

• Prevent a user from voting if he delegated his vote openslides-vote-service#225
• New vote delegation option #1883

[jsangmeister]

As discussed: The solutions described in the related issues will be split into two independent settings, one as described above and one to enable multi-mode.