Add new motion fields

[jsangmeister]

Two new fields should be added to motions: an editor and a working group speaker. Both can be (de-)activated in the settings.

Proposal for models.yml changes:

meeting:
  ...
  motions_enable_editor:
    type: boolean
    restriction_mode: B
  motions_enable_working_group_speaker:
    type: boolean
    restriction_mode: B

motion:
  ...
  editor_id:
    type: relation
    to: meeting_user/editor_for_motion_ids
    equal_fields: meeting_id
    restriction_mode: B
  working_group_speaker_id:
    type: relation
    to: meeting_user/working_group_speaker_for_motion_ids
    equal_fields: meeting_id
    restriction_mode: B

The restriction mode B is new for motions. Only users with motion.can_manage should be able to see fields with this restriction mode.

The actions motion.update and meeting.update must be updated accordingly to include the new fields. The motion fields can only be updated with motion.can_manage, the meeting fields analogously to all other settings.