Require our Obf base class just once in derived class.
Fix incompatibilities with different themes.

Other minor changes:

* Show a proper error message when authentication with backpack fails.
* Extend group menu via new plugin callback if possible.
* Fix port issue when authenticating to backpack.
Fix empty badge list when the categories don't exist in OBF anymore.
Fix duplicate navigation elements in group navigation in Mahara 1.9 -> .
Fix again group navigation.
Fix recipient list displaying incorrect group members.

When a group member has issuance privileges in an institution
but the member is neither group admin nor staff, the recipient
list (when issuing a badge) didn't display the correct group
members.
Fix several issues with group navigation and privileges.

- Fix Badges-link showing twice with newer versions of Mahara.
- Make database constraints work with PostgreSQL (Thanks to Mahara Team).
- Use Mahara's data directory to store the certificates instead
of plugin directory (Thanks to Mahara Team).
- Fix warnings when trying to add a menuitem when menu doesn't exist.
- Do not show the Badges-link if the member (with issuance privileges)
isn't yet a member of the group.
- Bump up the version number.