feature: edit a mirror entry in the frontend

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
improvement: for mirror editing use an object copy, which updates the current state after the server confirms at save

using a temporary object makes it more consistent and easier to cancel an edit and also make sure that once the object is saved the response from the server is used to reflect the current state (i.e. don't use the local state but the server state).

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>