Add additional class to sitewide notices table element

[hnla]

For some reason we have the same class messages-notices on both messages table loop & notices table loop messages-loop.php & notices-loop.php.

This causes issues trying to target the two screens and table displays without having to break out of the nested scss ruleset flow i.e #buddypress{table {} }

Preferable would be to remove messages-notices from the general user messages loop but this might have issues if styles have been built on this class; so alternative is add sitewide-notices to the notices loop class.

Patch adds this class to style on.

[r-a-y]

Yeah, I noticed this as well when I was looking at the twentyfifteen companion stylesheet and preparing some enhancements.

I'm okay with this going in.

[djpaul]

In 9863:

Templates: add new CSS class to Notices loop wrapper.

This lets themes target the Notices loop specifically. Previously, you could only use a selector that matches both the Messages and Notices loop.

Fixes #6424, props hnla