permissions for static placeholder editing
added tests to permissions for static placeholder editing
removed the STATIC_PLACEHOLDER_PERMISSION settings as suggested
Permissions for static placeholders, update

* Renamed is_flat to is_editable
* made render_plugin templatetag aware of the placeholder
* updated the render_plugin function to consider plugin.is_editable
added migration for static placeholder user permissions
moved migration
fixes to errors introduced in rebase
rebase and renamed db migration
Merge branch 'develop' into glomium-static_permissions

Conflicts:
	cms/templatetags/cms_tags.py
fix tests
Merge pull request #3381 from digi604/static_permissions

Static placeholder  permissions rebase