Ensure the beatmap set host's current username is consistent with the creator field of submitted beatmaps

[bdach]

This is the case in stable BSS. I initally avoided porting these checks as I must have seen them as pointless but after discussions elsewhere I realise that was probably an incorrect assessment.

Note that this may be seen as a bit strange / not necessarily make very much sense in isolation for guest beatmaps, but I'm choosing to ignore that for the time being, because guest support isn't getting added client-side immediately anyway. Not until guest difficulties are displayed properly all over (in song select and such).

This contains many test changes because it turns out that the one set we've been using for testing all over is the very rare exception to the rule - it was compiled from other people's works internally as part of a contest, so it is expected and normal that it has differing Creator: values across the set. But for the tests to pass, something has to give, so the archive has been changed here to specify a single creator across all difficulties. There were also several knock-on related test fixes that I will not elaborate on here because I'm not sure it's worth the time to do so.