This is a long-overdue change to make rbmotd work with Review Board 7

(and, more specifically, Django 4.2). This primarily deals with import

updates.
We now use Ink styling for the message banner, ensuring light and dark

mode work correctly. The close button has been moved to the right-hand

side of the banner, and accessibility information has been added for all

parts of the banner.

Tested on light and dark modes.
Checked the accessibility tree for the right information.