Resolve "Cannot edit users details"

Closes #869 (closed)

The problem was that /manage/users/users (the UI for local accounts only) while the API endpoint expects a federation username. In some circumstances, the two values can differ because we have a strict policy on allowed characters in the federation username, and strip everything but letters, digits and underscores. A user having a test.example username would actually get a testexample federation username.

cc @funkwhale/reviewers-python