Notifications: improve link management & customization

[imath]

• Redirection URL used can be improved using bp_get_notifications_unread_permalink() & bp_get_notifications_read_permalink() functions
• A filter is wrongly named bp_get_notifications_unread_permalink instead of bp_get_notifications_read_permalink

[imath]

There's also 2 missing variables for one of the filters of groups_format_notifications()

[imath]

In 13034:

Notifications: improve Action URLs management & customization

• Use bp_get_notifications_unread_permalink() and bp_get_notifications_read_permalink() to set the Notifications screens/actions redirections instead of rebuilding the redirection URLs from scratch. This lets Advanced users customize the redirection links if needed.
• Add 2 missing variables to the bp_groups_multiple_membership_request_rejected_notification dynamic filter.
• Edit the name of the filter used by the bp_get_notifications_read_permalink() function to be more consistent with the name of the function.

Fixes #8543