Domain group disco button

[BetaRays]

This adds the ability to do MUC channel search for domain groups when using muc_grouped_by_domain.

As this is currently implemented, using one of these buttons will set the default MUC domain globally, which can affect later uses of the global "Query for Groupchats" feature: I don’t know if this can have any unwanted effects yet (it doesn’t seem to be saved after logging out at least).

Before submitting your request, please make sure the following conditions are met:

• [ ] Add a changelog entry for your change in CHANGES.md muc_grouped_by_domain is only in 11, which is not yet released, so that might not be necessary.
• [ ] When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst not applicable
• Please add a test for your change. Tests can be run in the commandline
  with make check or you can run them in the browser by running make serve
  and then opening http://localhost:8000/tests.html.