Improve BP Messages/BP Nouveau Messages UI extensibility

[imath]

Working on BP Attachments, the BP Messages component and Nouveau's Messages UI needs to be improved in order to be able:

• to run custom code once the message is sent if some specific arguments are available -> Include the $r parameter to the 'messages_message_sent' hook.
• to remove attached media to a message if the user reset the message form.

[imath]

In 13369:

Improve BP Messages & BP Nouveau Messages UI extensibility

• Include a new parameter to the messages_message_sent hook to pass the messages_new_message() function parsed arguments.
• Fire a JavaScript event once the form to post a new message has been reset by the user.

Fixes #8768