Warn on account creation when username is adjusted

The user name can be adjusted due to various technical restrictions:
the first letter is capitalized, underscores are changed to spaces,
numerous other less visible changes happen. Some of these tweaks can
be unwanted by some users.

Generate a warning if that happens. If the user has JavaScript
enabled, the check happens entirely client-side – a little warning box
is shown, the user doesn't have to do anything. Otherwise the check
happens server-side and the user has to resubmit the form.

The way this is done makes it trivial to also check if the username
is invalid or already taken, so let's do that. It also means that we
can't check for all error conditions, e.g. these enforced by
extensions – that is still handled server-side. (Client-side we
intentionally never say that whatever the user typed in is valid – we
only warn when we know it's not.)

API behavior is unchanged.

Co-Authored-By: umherirrender <umherirrender_de.wp@web.de>
Co-Authored-By: Bartosz Dziewoński <matma.rex@gmail.com>
Bug: 34447
Bug: 61416
Change-Id: Ic461a5e597ad71b854dc65bbf8a395c0f55d1fc3
Merge "Warn on account creation when username is adjusted"