[kcm/users] Move to plasma-workspace

As discussed in teams/plasma-mobile/issues#65, moving the users KCM to plasma-workspace allows us to share code for a kcm to change passwords, fingerprints, etc. (currently at Plasma Mobile we have a separate kcm for that).

The intent is to eventually work on a mobile interface for the kcm after this is merged (of course, trying to share components as much as possible).

plasma-workspace MR: plasma-workspace!1133 (merged)