homework: also apply uid shifting when changing passwords/resizing/updating home...

homework: also apply uid shifting when changing passwords/resizing/updating home areas

This adds uidmap shifting also when resizing/updating/changing
passwords. Prviously I thought we didn't have to, because the user is
not going to access the uidmap if we only quickly activate the home
area. But this thinking is wrong, because the three operations will
result in an update ~/.identity fie to be written, and we should do that
with uidmap applied, so that its ownership maps down to nobody below as
intended.

Fixes: #21441