Apply 'bp_get_group_permalink' filter during digest formatting.

Plugins like BP Group Hierarchy hook into this filter in order to
manipulate the group permalink.

bp-activity-subscription-digest.php

@@ -286,7 +286,7 @@ function ass_digest_fire_test() {
 function ass_digest_format_item_group( $group_id, $activity_ids, $type, $group_name, $group_slug, $user_id ) {
 	global $bp, $ass_email_css;
 
-	$group_permalink = bp_get_root_domain() . '/' . bp_get_groups_root_slug() . '/' . $group_slug . '/';
+	$group_permalink = apply_filters( 'bp_get_group_permalink', bp_get_root_domain() . '/' . bp_get_groups_root_slug() . '/' . $group_slug . '/' );
 	$group_name_link = '<a href="'.$group_permalink.'" name="'.$group_slug.'">'.$group_name.'</a>';
 
 	$userdomain = ass_digest_get_user_domain( $user_id );