Groups widget: Set the `per_page` argument in bp_has_groups().

Previously, the `per_page` argument was not passed to bp_has_groups().
This meant that it wasn't possible to show more than twenty groups for the
groups widget.  This commit addresses this.

Commit also switches the querystring to an array when setting up bp_has_groups().

Fixes #5695.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8480 cdf35c40-ae34-48e0-9cc9-0c9da1808c22