Fix for python 3 in plugin_pool
Merge pull request #3355 from yakky/feature/fix_py3

Fix for python 3 in plugin_pool
fixes #3348
Merge pull request #3357 from digi604/fix-3348

fixes #3348
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