perf(upgrade): ajaxifies data directory migration

This allows upgrade.php to complete more quickly by moving the user data directory migrations to a separate page with an ajax process.

Fixes #6202
Replaces #6336

Changes to Steve's original PR:
 - Uses the generic XHR upgrade view and js
 - Uses the ElggUpgrade object
 - Fixes a bug that made it upgrade only the first ten users
 - Rebased with latest master