I added a route that let the logged in user to add a tag to the database
along with its controller and template. This patch is working but has
two flaws. The first is that the user is redirected to the home page and
it doesn't save the corresponding tag to the user's app.
It should do the latter and then redirect the user to the webapp update
template.

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