Custom profile fields allow for shortnames containing mixed character
casing, which should also be supported within the user fields API
when retrieving said custom fields.

Given that the DML libs convert all selected columns to lowercased
equivalent, we need to do the same when requesting custom fields to
ensure calling code doesn't try to access `->profile_field_Foo`
property when the DB returns `->profile_field_foo` instead.

Pass `$PAGE->subpage` value in AJAX request, which is required on
some pages (e.g. user dashboard) for loading existing blocks.

This commit is UNRELATED to the MDL-72911, but we need the
@skip_interim tag removed to one scenario that was causing lots
of noise and failures @ CI infrastructure. This tag can now be
removed because these failures were fixed a days ago.

So we are using MDL-72911 (this extra commit) to remove that tag
(sorry for the noise).

A new setting has been added to config_plugins to store the last
preset identifier applied.

Exclude the last node in a breadcrumb change from auto-removal if no URL provided.