I updated the tag views by importing the `Tag` model provided by the
Taggit plugin, deleted the `WebappTag` model and `TagForm` form that are
obsolete.
I updated the `WebappUpdate` view to follow the recommendation given by
the Taggit plugin documentation. The `TagList` now uses directly the
`Tag` model and send both the tag fields and a count of each tag
occurence.
I removed the `TagAdd` list that is obsolete. I updated the
`TagWebappList` that returns simply a list of all `Webapp`objects for a
given tag.

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