Warn username with full stop (period) can be problematic

Proposal

We have long time open issues such as not being able to properly set up pages #20184 (closed) if a username has a period.

As a short term solution, I propose adding a warning if a user enters a period in the username when:

1. registering
2. changing their username

While I'm not a fan of creating this as a "workaround" for another bug, changing a username can have a lot of side effects and may not be a feasible workaround, and the existing bug issue is a long standing one.