Implement time limits for motions

We need to implement time limits (per meeting) for different types of motions.
These time limits should be implemented as a new model within the meetings: time_limit.
It should include the following fields:

• name: a string representing the title of the time limit
• meeting_id: relation to corresponding meeting
• begin: a timestamp marking the point from which a user is allowed to carry out the associated actions
• end: a timestamp marking the end of the time period
• type: a multiselect specifying the actions which the time limit restricts.
  • the selectable options are: create motions, update motions, support motions, create amendments, update amendments, support amendments

There ought to be corresponding new actions time_limit.create, time_limit.update and time_limit.delete that allow the editing of these models.

All actions that involve creating/updating motions or editing the supporters of a motion should have their permissions changed to consider the time_limits in the following manner:

• If there are no time limits concerning the attempted action, proceed as usual.
• If there are time limits concerning the attempted action, normal users can only perform that action while the current time is within one of those time limits.
• Given that certain members staff should be able to edit motions after the fact for organizational reasons there should be a new permission can_ignore_time_limit that causes this time limit check to be skipped.

Which fields of the motion actions will be affected specifically will have to be discussed at a later date.

TODO: What will have to be done in the autoupdate service?

Start by reviewing the existing meeting and motion models, their action permissions, and the autoupdate service. Clarify which motion-action fields are affected and how time limits should interact with autoupdates before implementation. Done means the time_limit model, create/update/delete actions, permission bypass, and affected motion and amendment actions are specified and covered.