Merge fixes by Douglas Cerna:
- Made the gradebook to stop creating a default worksheet if there are report sheets deployed (https://launchpad.net/bugs/579720)
- Handled hidden persistent score systems in Report Activity forms
- Fixed failing ranged score system for report activities when using non-english views (https://launchpad.net/bugs/581825)
- Improved due filter message id in the gradebook overview
- Fixed typo (https://launchpad.net/bugs/582436)