Add ability to dismiss server banners #3937

Putting feelers out for interest on this, willing to implement it, but want to gauge interest first. At my workplace, we use Github Enterprise, and our admins are keen to use the site-wide banner in order to reiterate policies at the cost of screen space.

This is on the top of every page, including the login page, and banners are a core github-ent feature, not custom css/html. I'd like to be able to dismiss it if I've already been welcomed 100+ times before.

To that end, I'd like to put in a new feature that does the following:

• Checks for the presence of a enterprise-wide/server-wide banner
• Make an 'X' on the side of the banner
• Upon clicking the 'X', cache the text of the banner (or a hash of it) in per-site local storage and hide it.
• With an existing cache from dismissing a banner before, hide the banner on page load unless it does not match the cache. That way, actual useful information (eg, an upcoming maintenance) is not hidden unless you dismiss it again.

This is not seen by most github users, being rather gh-ent specific, but I feel it's unobtrusive and useful enough to be included here.