Messages notifications filters: it needs some clean-up!

[imath]

The dynamic filter 'bp_messages_' . $amount . '_new_message_notification' is including 6 parameters when the notification format is string and the exact same filter is including 7 parameters when the notification format is array.

We need to deprecate this filter in favor of a new one to clean up this mess (I'm probably the one who put it in the first place, btw).

[imath]

In 13035:

Make the new_message notification filter consistent across formats

The 'bp_messages_' . $amount . '_new_message_notification' dynamic filter is including 6 parameters for the string format and 7 for the array format. This makes it difficult to use for Plugin developers.

To make things more consistent across formats, we are deprecating this filter in favor of a new dynamic filter 'bp_messages_' . $amount . '_new_message_' . $format . '_notification'. FYI, possible filter names are:

• 'bp_messages_multiple_new_message_string_notification'
• 'bp_messages_single_new_message_string_notification'
• 'bp_messages_multiple_new_message_array_notification'
• 'bp_messages_single_new_message_array_notification'

Fixes #8546