ChatControl: Add "Block Contact" menu item
This adds a "Block Contact…" menu item to ChatControls, which allows to either block or block and report a user.
I explicitly used XEP-0191 Blocking Command only, since Privacy Lists are deprecated.
I also simplified strings used in the blocking dialog (and had to adapt the ones used for blocking from the contact list).
In order to add spam reporting to the contact list blocking feature, we should probably remove Privacy Lists usage there first.