In order to add a link to a Jupyter notebook I had to perform several
changes.

The URL field provided by django is unable to manage git url type such
as `git@example.net:user/repo` therefore I changed to a CharField.

I updated the field validator for the field `notebook_gitrepo` and use a
generic regex. At the moment I ensure that the url corresponds to
`git@something`. It's a bit light but it's a start.

I added the jupyter hub link for an authentified user that is the owner
of the application in public webapp list.

I fixed as well a missing closing tag.

The placeholder attribute in the webapp update form on the
`notebook_gitrepo` form was wrong. I fixed it and updated the content in
order to specify the type of url expected.

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