[#1251] Update IResourceView interface.

Use 'view' instead of 'preview' and update docstrings.
Update code that uses IResourceView plugins
accordingly.
[#1251] Update ckanext.imageview to work with new
IResourceView changes.

Use 'view' instead of 'preview'.

Move 'form_template' to top level method.

If no image_url specified, try to view the resource url.