Create multiple chat rooms in one request

[evdherberg]

New endpoint to create multiple chatrooms in one go. Fixes #127.

[guusdk]

Would it make sense to group the results by result type? That is most likely how the returned object is going to be consumed anyway. I am easily convinced otherwise though.

In any case, please add:

• a reference to the changelog to the issue that you fixed.
• @Schema annotations to the newly added entities, for documentation purposes (as explained in For all entities, add schema annotation to all (XML) elements #133)
• the new endpoints to the documentation provided in the file readme.md.

Afterwards, you can regenerate the readme.html file as described in the commit message of 315922b, but lets do that just once (to include all changes from all of the PRs that you are creating). This will prevent merge conflicts.

[guusdk]

Thanks! One issue that popped up during testing is that rooms that already exist can be 'created' again. This, however, is pre-existing behavior. I have created a new issue for this problem: #140