Validate or create the top-level "Circles" space

Especially for self-hosted Matrix accounts, the account may not have the correct Circles space hierarchy when the app first starts.

We currently validate that all of our Spaces are there, but we fail completely if any of the major spaces are missing.

We need to take this one step farther and at least give the user the option to create any missing spaces.

This requires some new code to run behind the scenes, and it requires a new state in the UI where we will show this dialog before we "boot up" the main Circles interface.