Importer: display image bases in red if they are not accessible via HTTP

The confirmation dialog of the importer now shows whether an image base
is accessible via HTTP. The image base is marked red if not and it is
marked green if it can be accessed.
Importer: fix typos
Importer: mark group permission setting as not required
Importer: fix small bug in confirmation template
Importer: fix bug that could break HTTP accessibility test for stacks
Importer: use correct file extension for HTTP accessibility test
Importer: add option to select user permissions
Importer: only use color in HTTP test output if test fails
Use default template context processors of Django

Template context processors provide templates with a basic context like
the path of static files. Up to now the default processors were not (!)
used. That confused some Django applications like django-guardian which
couldn't refer to some static files because of this. This commits
ensures that Django's default context processors are in use and allows
to easily add custom ones.

Additionally, an old and not used custom template context processor was
removed. Now there are only the defaults processors in use.