Limits API

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
There is currently no way to impose artificial limits (rooms-as-type per user, users per room-as-type, child rooms per parent room) and communicate them in a portable manner. We need a way to impose and communicate those limits. Those kinds of limits are useful in company, government, school and limited traffic cloud use cases. Several MSCs need to be drafted for each of those three kinds of limits.

**Describe the solution you'd like**
* Per-room limits (which might be different for each room type), where room members with tombstone permissions can change.
* Rooms-per-user (which might be different for each room type) limits, where server admins can change.
* Child rooms per room limits, where the members of parent room having both tombstone and attach/detach room permissions can change.
* A reliable limit query API, where any user can query the limits applicable in a given situation.

Start by reviewing the three proposed limit categories in this issue and the requirement for a portable query API. Draft the necessary MSCs so the limits, who may change them, and how any user queries the applicable limits are clearly specified.

Written by the indexing model from the issue text.