A common design pattern is to place a banner at the top of the page, but

we've left the details of that to each extension needing a banner. The

rbmotd and Power Pack extensions both provide their own banners,

choosing their own placements and styling. This creates some

inconsistency.
This change adds a formal definition, stying, and placement for page

banners. These are placed in a new hook point just below the navigation

bar.
A reusable component-based template, ui/components/page-banner.html,

makes it easy to render a page banner. This contains some "parameters"

that can be populated either by inheriting from the template and filling

out blocks or by passing arguments in an {% include %}.
This makes it easy to use this with a TemplateHook or page state

injections.
Documentation, a Storybook story, and updates to the Read Only Banner

will be coming in follow-up changes.

Unit tests pass.
Ported rbmotd over to this, and used it for the new Power Pack banner.
Tested in desktop and mobile modes.