Previously, random selection would always be done at a *set* level. The
final operation of a random action would be "select the user's
recommended difficulty from this randomly selected set".
This makes no sense when sets are not grouped together at song select.
In fact, it is completely broken with the previous commit which adds
group-isolated random support – if we're grouping by difficulty and the
user's recommendation is not in the current group it would throw the
user into another group unexpectedly.
This fixes the issue by splitting out the random implementation into two
separate pathways depending on the carousel display mode.
This is a minimal implementation in order to keep moving forward.
For simplicity I've copied over the old implementation verbatim. Note
that this is beatmap*set* based randomisation, which means that when
panels are split up by difficulty, it is still randomising by set (with
the difficulty choice being left up to the user recommendation system).
I think this is what we want, but if it isn't, any changes can come
later.