It was painful to change a webapp ownership because error messages
weren't displayed in case of failure or success. Now if the new owner
doesn't exist in the database, the user has an error message asking him
to enter a valid email.
In case of success, the user is redirected to his webapp list with a
success message remembering him to which user he transfered the app.

Resolves: #167



Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>