Add redirect_to URL param support to groups_screen_group_invites()

This allows custom redirects after the acceptance/rejection of a group
invitation, for increased flexibility when generating Accept/Reject links
throughout BuddyPress.

See #5032

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7440 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
BP_Groups_Member::check_is_banned() should be marked static

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7441 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Improve UX flow around overlapping group invites and requests

This changeset introduces a number of improvements to the flow surrounding
invitations and membership requests to private groups:

- When a user has previously requested membership in a group, add a note to
  that effect when the user is being invited to the group via the Send Invites
  interface.
- When a user has previously requested membership in a group, automatically
  accept that invitation and add the user to the group when an invitation is
  sent by an existing group member.
- When a user has previously been invited to a group, buttons in the group
  directory and in the group header should read Accept Invitation instead of
  Request Membership

The end result is a more logical user experience with regard to joining private
groups.

Fixes #5032

Props Mike_Cowobo

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7442 cdf35c40-ae34-48e0-9cc9-0c9da1808c22