webadmin: Changed "Assign/Unassign" to "Manage"

The dialog lets the user do more than just assign/unassign, so changed
the wording as well.

Change-Id: Id5b8380c72c4c225260f838a4e7e23228adcf542
Bug-Url: https://bugzilla.redhat.com/980066
Signed-off-by: Lior Vernia <lvernia@redhat.com>