Implement motion_supporter model

[luisa-beerboom]

Closes #3106
See OpenSlides/openslides-meta#330

Info for client devs:

1. This PR removes the motion.support_self action, use motion_submitter.create and motion_submitter.delete from now on
2. Adding a meeting_user id-list for supporter_meeting_user_ids in motion.create will still create supporters
3. There is no longer a possibility to edit supporters with motion.update, use motion_submitter.create and motion_submitter.delete
4. Take note that the new permission rules for motion_submitter.create and motion_submitter.delete differ slightly from how it was with the previous system due to the fact that the permission requirements were merged together (see docs for rules)

[hjanott]

Found some questions and things to improve.

[hjanott]

I see that you moved this file from motion/test_set_support_self.py. Or is this a github glitch? Will this lead to any complications when integrating this change into rel-db? Especially considering the current changes mixing with the rel-db ones? https://github.com/OpenSlides/openslides-backend/pull/3085/files#diff-3cd46c8071338c282fc2d1f68ac564beeab783c63c3b820b37b35f9881978b25

[luisa-beerboom]

Had to split the test_set_support_self tests between this and the create action test. Idk about how github's going to treat this in relation to relDB, though I guess it may have to be changed for it

[hjanott]

Please resolve conflicts.