Replace internals of BP_Group_Membership_Requests_Template with BP_Group_Member_Query

This brings us more in line with the Invites template functions, and is another
step toward centralizing all of our group queries.

The change also allows for true pagination. This changeset also adds the 'page'
parameter to the stack.

See #5440

Props dcavins

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8069 cdf35c40-ae34-48e0-9cc9-0c9da1808c22