allow to dismiss announcement bars

[drammock]

#1354 adds a version warning bar in addition to our announcement bar. It would be nice if users could click an X to dismiss the announcements/warnings (they take up quite some space at the top of every page). Not sure how easy it is to persist that setting (or if we even want to do that), seems like options ought to be:

1. show banner / require clicking X on every page
2. keep track if banner was dismissed for all pages within that site version
3. keep track if banner was dismissed for all pages in that URL domain (all docs versions)

I lean toward the simplest (option 1) but I'm open to being convinced otherwise.