Moving sidebar outside of the main edit form so plugins can handle their own form data independently

Changing plugin base models for a more intuitive understanding of what types of models should exist. The image model should maintain revisions of itself to avoid a difficult process when replacing images.

Fixed render_to_string parameter (context was being passed in without using 'context_instance' kwarg)

Adding dependency on sorl-thumbnail

Message after uploading image. Do not redirect after plugin form submit as changes to article text and title are lost.

Conflicts:
	wiki/conf/settings.py

Corrected parameters on render_to_response. Also fixed login url resolution when ACCOUNT_HANDLING=False.