Feature: LDAP users could be deleted over the User-Settings-Web-UI

In previous versions ( <=9.1) the LDAP users could be only be managed over the occ command and cron job:

There was a background job that has checked 50 users every 51 minutes and they were marked as remnants, long and tedious process that took some months to detect all users.
After that, the administrator could run over occ command the delete user command to remove those users. The administrator didn't have a quick overview or search for those users.
No LDAP user was able to be removed until it was marked in the DB by modifying the remnant state.

Some users wanted to have a "fresh start" being removed and then clean up all unwanted information.

In ownCloud 10 we introduced the new occ user:sync command that detects all users at once and it can be marked as disabled to keep the information and transfer to other users.

It would be an advantage, that the LDAP users can be removed over the User-Settings-Web-UI because the users can be scrolled or searched as all other users. then the administrator can choose to remove them.

An additional use is when a user asks to be removed to have a clean storage (it's asked frequently), the administrator can remove the LDAP users (files, old-shares, calendar, contact's, etc.) and once he logs in again can have a new clean environment.

As security, to avoid removing users by mistake can be added that the non-disabled users can't be removed.

@hodyroff @pmaier1

Start by tracing the User-Settings-Web-UI user listing and the existing occ user:sync and user-deletion flows. Determine how LDAP users are identified and how disabled status is represented. Done means administrators can find and remove eligible LDAP users in the UI while non-disabled users are protected and the associated cleanup behavior is preserved.