Introduce bp_the_profile_field_ids(), and use as appropriate throughout BP

The POST routine run when editing one's xprofile-powered profile (or
registering for an account when the registration form includes xprofile fields)
works in part by sending a list of field IDs in the 'field_ids' <input> element.
These field IDs are then used to decide which POST keys to look for when saving
submitted profile values. Historically, the function powering this field_ids
element has been bp_the_profile_group_field_ids(), which pulls up only the
field IDs from a single field group. Normally this works fine, because the
typical Profile > Edit screen only allows for the editing of a single field
group at a time. However, the introduction in BP 2.0 of Settings > Profile
demonstrates that this technique breaks when attempting to save more than one
field group at a time.

The new bp_the_profile_field_ids() replaces this logic by pulling up all the
field_ids from all the groups in the $profile_template global object.

In order to provide for backward compatibility with themes that may have
overridden the modified templates (in particular, members/single/settings/profile.php),
we add logic to the Settings > Profile save routine that looks for any
submitted profile fields that are not present in the field_ids array.

Fixes #5666

Props boonebgorges, r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8819 cdf35c40-ae34-48e0-9cc9-0c9da1808c22