Sort after display name + LDAP server side sorting

LDAP server side sorting added. To take real effect, this PR changes the sorting on the Users page to sort according to the display name instead of the internal/login name. The internal name from LDAP users will be unreadable UUID like AF926DEA-E6B4-49C6-9A14-D2E918C2F9B2 so it does not make sense to sort it after this.

Hope that is okay. Adresses an issue stated in https://github.com/owncloud/core/issues/1609 It does not do natural sort, however.

What do you think @jancborchardt @tanghus @schiesbn @karlitschek others?