Add template tag for 'is_invited' & update private group messages in bp-groups-template

[hnla]

Patch adds a template tag check in bp-groups-template.php to return bool on whether they have had an invite to join private group.

Updates the groups status message variable to check if user has invite and display a new message stating they have an invite.

The message update just makes for a more helpful message if a user enters a private group they may have been invited to but not realised by replacing the message that they must request membership, as it's already in motion.

Template tag was just to provide capability of running a check in templates but may not be that useful or flexible the way I've written it.

[hnla]

New template tag & update home screen message

[boonebgorges]

In 8411:

Introduce bp_group_is_invited(), and use it to improve private group message for non-members

When non-member visits a private group of which she is not a member, she now
sees a message that tells her to accept her pending invitation.

Fixes #5510

Props hnla