Addresses#1384

Using essentially the same approach as was used for the version warning banner in#1763

Some questions for the announcement banner compared to the versions:

• Do we also want to only remember it for 14 days, or just forever for a certain announcement?
• Ideally the stored dismiss state takes into account the content of the announcement, so that when you add a new announcement, all users see it?
• I am assuming that for a general announcement banner, we donot want this to be version specific (i.e. remember it separately per version)

Note: I only tested it with a remote announcement that gets injected, not yet with a local snippet.