Per-user room and session limits

This issue has been migrated from [#9707](https://github.com/matrix-org/synapse/issues/9707).

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**Description:**

We need an ability to limit the number of rooms and/or open sessions per user.

Spec proposal: https://github.com/matrix-org/matrix-doc/pull/3021

@Sorunome said that x-per-user does not need an MSC:

> how / why?
> soru can see x-per-users be helpful in commercial aspects, but not users-per-x
> conveniently, x-per-users is an implmentation detail
> x-per-user is an implementation detail and shouldn't need an msc

As this only affects the users of a given homeserver, it can safely be implemented ahead of standardisation.

Start with the linked Matrix spec proposal and review Synapse's existing room and session handling to identify the relevant entry points. The work is complete when a homeserver can configure and enforce separate per-user limits for rooms and open sessions, with coverage for the resulting behavior.

Written by the indexing model from the issue text.