Introduce permissions, restrict certain actions to editors and admins

https://www.pivotaltracker.com/s/projects/1015742/stories/65958732

This change:

- introduces 3 roles (viewer, editor, admin)
- restricts the viewer role to read-only actions
- hides the UI elements from the user if they don't have the appropriate role

This change should not be released until [each existing Maslow user receives the `editor` permission on production](https://github.com/alphagov/maslow/pull/96).