organisation: transform project list into a project gallery

The project list was just that, a list of project names and their
descriptions. Some projects have a banner image which can help
them stand out so the list has now been changed into a more
gallery-like list where banners are shown where possible.

The gallery can be filtered by popular projects (marked by admins
to be featured, not algorithmically by number of click or such),
by themes, and by project types.

The filter solution is server generated using url parameters but
it can later be done via javascript and a project api to avoid
page reload... just not now. Now we only want a project gallery.
Merge pull request #147 from okfn/project-gallery

Transform project list into a project gallery