Convert XProfile admin AJAX to REST API

[boonebgorges]

The existing AJAX in the XProfile admin area is a good first candidate for conversion to the REST API. See ​https://buddypress.trac.wordpress.org/browser/tags/4.1.0/src/bp-xprofile/bp-xprofile-admin.php#L592, ​https://buddypress.trac.wordpress.org/browser/tags/4.1.0/src/bp-xprofile/admin/js/admin.js#L228 for some examples.

[boonebgorges]

I've done some in-depth review of the existing AJAX here, and I don't know that there are any easy wins. Most of the interface does not, in fact, use AJAX. The only places where AJAX is actually used are as follows:

1. Updating field order within a group. But this doesn't neatly fit with the REST endpoints - we need a bulk action. Probably best kept as a custom AJAX endpoint for now.
2. Updating option order within a field. Same argument as 1.
3. There's JS and PHP that's meant to power AJAX deletion of profile field groups, but it's broken in a couple of fundamental ways: there's no 'confirm' message (as there is designed to be), there's no success/failure when performed asynchronously, and there's no preventDefault(), so the delete always takes place via a page refresh anyway. This could use fixing, but it's difficult to pick apart in this context.

As such, I'm rescinding my initial suggestion that this is a "good first candidate" :) Moving to the REST API would be part of a more substantial rewrite.