Set the attribute 'editable'

This makes many forms redundant that were only a ModelForm with an 'exclude'
list. This is now done in the model. Fixes #17
Improve the banner for guest users

The banner is also shown for logged out users and has a link to create sample
data (doing it always in the middleware is too slow). This changeset adds
'nofollow' attributes to links, to keep search engines out. Fixes #19
Remove unneeded templates, fixes #20
Merge