Allow putting the app ID in the password for bot passwords

Allow putting the app ID in the password for bot passwords

Bot passwords allow backwards-compatible login (with grants, for API
usage only) with "<real username>@<botname>" for username plus a
random-generated password.
This doesn't work well with some bot frameworks (including Pywikibot,
the most popular one) which assume that the text that goes into the
username field of the login API is the username that they will be
logged in with afterwards (and so the @-postfix causes all kinds of
errors).

Since the goal of bot passwords is compatibility with old unmaintained
API clients, this patch adds an alternative format which does not
cause problems with old bots: use the username normally, and use
"<botname>@<random-generated password>" as password. Since this is
technically a valid normal password, there is some ambiguity, but
bot passwords have a distintive format so it's easy to check and it is
extremely unlikely that someone would use the exact same format for
their normal password; and if the bot password login fails we can
simply retry it as a normal password, just in case.

Bug: T142304
Change-Id: Ib59a6fbe0e65d80d5e7d19ff37cec5e011c00539