A user can manage one app (several later). In order to be to do that I
created two views. One to list the apps belonging to him, another one to
edit a given app. Two routes have been created as well to perform these
actions. Two forms and templates have created as well and modified to
correspond to the current graphical chart. The user menu and the public
webapp listing have been updated to show the buttons to list or edit the
apps wherever it applies.

The model of `Webapp has been modified as well. A first modification was
about the `private` that was an integer. Because the value can be only
true or false, I changed it to a boolean. A field has been added called
`notebook_gitrepo` that corresponds to a URL to a git repo and used to
call the Jupyter hub and run the notebook.

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