Add alert banner above WP Dashboard widgets

[curtismchale]

Issue

Allow Kevin/Luke to add a top banner above the WP dashboard widgets that goes full width for Alerts. This should likely also show up on the my.proudcity.com dashboard for users and be controllable from either my.proudicity.com or via some type of RSS/JSON feed as suggested in #2565

Should also show up at the top of the new PC dashboard #2567 above all sites.

[lukefretwell]

Added to prototype sites page: https://proudcity.github.io/prototypes/sites/