MUC occupants badges: displays short labels, with full label as title.

[JohnXLivingston]

By default, we take the first letter of all badge, and only display this letter (full badge name is set as title and aria-label for screen readers). If there is ambiguous letters, we use 2 (or 3, ... up to 4) letters.

• Add a changelog entry for your change in CHANGES.md
• When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst
• 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.

[JohnXLivingston]

Tests failed on the CI, but seems not related to my PR. Tests are ok on my computer.

[jcbrand]

Tests failed on the CI, but seems not related to my PR. Tests are ok on my computer.

If you rebase on to the master branch then this should be fixed.