Pagination in group homepages does not work properly

[Son Nguyen]

Version: master (15.10), 15.04
Browser: any
Platform: any

The pagination of the page lists in the group homepage does now work properly when changing the sort option.

One scenario:
1. Login as a group member, create 7 pages, and share them to the group.
2. Login as the group admin, edit the group homepage, configure the "Group pages" block, change the "Sort shared pages and collections by" to "Most recently updated", and save the group homepage
3. Login as a group member, open the group homepage, navigate the list of shared pages
At first the list is sorted by "most recently updated", when click to next page or last page, the list now is sort by page title.

The bug also related to https://bugs.launchpad.net/mahara/+bug/1454458.