[#1298] Fix activity and about organization pages

The controller methods didn't use `self._action` which will use the
appropiate `group_` or `organization_` action depending on the group
type.

The `organization_activity_list_html` action hadn't been used at all
until now, as there was a syntax error.
Merge branch '1298-fix-orgs-activity-about-page'
Merge branch 'master' of github.com:okfn/ckan into 847-new-theming-docs