Simplify Theming

[allo-]

Create a good HTML template and delegate all theming to CSS files.

[allo-]

related: https://github.com/allo-/feedjack/issues/13

[allo-]

[ ] Theming via a path to a CSS-File in a config.
[ ] Full themes via additional packages with templates overriding HTML files using the TEMPLATE_DIRS search path. They can extend the default theme using django-apptemplates

[allo-]

CSS themes are reasonable, but there is nothing which would prevent full themes bringing their own base.html and all other needed files. They may be put in the TEMPLATE_DIRS by the admin or installed via pip as feedjack-theme-x package, as long as render("feedjack/{0}/file.html".format(themename)) works.

Still using the formatstring seems a reasonable choice to allow per-site and per-user theme choice.

Maybe the themes will be moved into a ``themes subfolder to have more general html snippets in the main path.

Old themes should be adapted, once the template and url structure is fixed again: https://github.com/allo-/feedjack/issues/18.