[BP Nouveau] Feedback messages on error/info/success about just posted activities are not styled

[imath]

When an activity failed to be saved, there's a feedback message displayed at the bottom of the form#whats-new-form see the yellow highlighted area in the screencap below:

@hnla to replicate, you can post an empty activity from the site wide activity directory page ;)

[hnla]

@imath Thanks I'll take a look at.

[hnla]

In 12011:

Nouveau: update Activity post form error display - structure & classes

Commit addresses lack of styling for activity post form empty submission error message.

Adds .bp-messages.bp-feedback classes to json return in activity-post-form JS.
Adds span icon element to Backbone template error rendering markup to display Nouveau's message feedback icons.

Props imath

Fixes #7770