Handle "xBUTNUMBER" xlogin cases better

[Toaster192]

Either add a hardcoded check for xBUTNUMBER API calls that in case the request doesn't return anything, tries again with just the BUTNUMBER ?
Otherwise we could also handle this via #1147 by forcing reverify of people with this kind xlogin.

There is also the "xp" FP login bullshit but I don't think there is any obvious solution to that one and it should affect only a tiny amount of people (or I guess it doesn't matter to us as those logins never could get to the database in the past as FIT generated their own login and now they are forced to use BUTNUMBER instead so we don't care that their VUT xlogin doesn't match the FIT xlogin). If there are other weird exceptions we should track them in this issue.