Remove all references to the 'ediable' property of a stack from front end

This propery is of no more use since we moved to Django and its per-object
permissions.
Remove 'editable' property of projects from back-end

This removes some permission queries when loading all projects. The
'editable' property is of no use anymore and can safely be removed.
Merge pull request #658 from acardona/remove-editable-property

Remove editable property