Fixed notice at the bottom of the screen

[brcsw]

We'd like to add an always-on notice to every page. Desired properties:

• Shows up at the bottom of the viewport
• Always visible (don't need to scroll to see it)
• Doesn't cover other content

This is similar to Banner, but banners are at the top of the page, and they're too prominent. Plus we want to be able to use banners for announcements without losing our notice. We explored whether we could use a custom script to add fixed div at the bottom of the viewport, but it covered up other content. We tried having the custom script add some margin-bottom to an outer container <body> or <div> to make room for the fixed notice, but the margin seemed to have no effect. We enhanced the custom script to dynamically resize some of the other elements. This works, but is brittle. We'd like to have a supported, non-brittle way to do this.