Add new meeting specific setting

[Elblinator]

Context: OpenSlides/openslides-client#4170

Wanted:
A meeting needs a new setting to make the background from the motion-metadata transparent in the projector.

Recommendation for a name: motions_hide_metadata_background
It should be a boolean.

The backend only needs to enable/disable this setting.

This setting should be optional, there should be no migration.
If the setting is none the client should pretend like the setting is false (I am not sure if that is relevant for the backend-issue).

[Elblinator]

I updated the name to motions_hide_metadata_background