Groups: Introduces new function - groups_get_invite_count_for_user().

Previously, BP_Groups_Member::get_invites() did two queries -- one to
get a paginated array of group invites for a user and the second to
get the total group invite count for a user.

This commit:
- Splits up the total group invite count query into a separate class method - BP_Group_Member::get_invite_count_for_user()
- Adds a wrapper function - groups_get_invite_count_for_user() - and uses it in the BP_Groups_Component class where the invite count is only needed.

The group invite count will be cached in a future commit.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8057 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r8057

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8058 cdf35c40-ae34-48e0-9cc9-0c9da1808c22