Create a banner-like ability for important announcements

[samccann]

We have the ability to have a top banner on the docs pages and the forum. This is a request to add something similar to the website front page so we can announce important things like AnsibleFest/Summit, Community days, etc.

[x1101]

Please assign this to me. I'm working out how to do this, and getting a proof-of-concept of it going.

[x1101]

Should the banner "stick" to the top of the window, or only be visible at the top of the page?