[Fix Bug 907658] Changing GROUPS_PER_PAGE setting to 20.

See [Bug 907658](https://bugzilla.mozilla.org/show_bug.cgi?id=907658) for reference.

In the refactoring of `apps/groups/views.py` there was a new setting added called `GROUPS_PER_PAGE` which replaced `PAGINATION_LIMIT_LARGE`. Groups and functional areas list pages were only displaying one item, since that settings was set to that value.