Change default per_page for bp_group_has_invites() to false

This change is necessary for backward compatibility, as the old internals
did not have pagination parameters. See eg bp-default.

See #5423

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8062 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Introduce pagination for Send Invites page in single groups

For better support for up-to-date pagination, the AJAX for group invitations
now refreshes the entire invitation section, rather than simply adding single
items to/removing single items from the DOM. This change requires a new
template, groups/single/invite-loop.php, which is the item loaded by the AJAX
request. Backward compatibility with bp-default-style themes is maintained in
the JS.

Fixes #5423

Props dcavins

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8063 cdf35c40-ae34-48e0-9cc9-0c9da1808c22