Messages: AJAX replies should use the message loop

[r-a-y]

When making a reply to a private message thread, our AJAX message output should use the message loop and all related message template functions.

Currently during AJAX, we do a workaround fix for zebra-striping (see r6548) and we do not use native template functions such as bp_the_thread_message_css_class() (see r8604) or bp_the_thread_message_content() (see r2191).

This is because we're not using the message loop correctly when rendering the AJAX content.

Attached patch emulates the message loop to ensure that the current message in the loop is correctly set. This allows us to use our message template functions without resorting to workarounds.

[johnjamesjacoby]

A long-standing enhancement in the making.

Nice work!

[r-a-y]

In 9838:

bp-legacy: For AJAX private message reply, use the message loop.

When making a reply to a private message thread, our AJAX message output
should use the message loop and all related message template functions.

Previously, we did a workaround fix for zebra-striping (see r6548) and did
not use native template functions such as bp_the_thread_message_css_class()
(see r8604) or bp_the_thread_message_content() during AJAX.

Fixes #6321.