Banner indicating high request volume to all users of the registrar

Issue description

Problem: Due to unprecedented volume of inbound.gov, domain requests, domain requesters are experiencing long lead-times awaiting adjudication of their requests. This leads to inbound contact, often seeking status of a pending domain request, which consumes helpdesk resources, and time.

Solution: At a certain threshold of pending domain requests, display a banner for all users of the registrar, including non-authenticated users on get.gov, indicating that the system is experiencing a high volume of requests and that the time to approval may be longer than normal.

Acceptance criteria

[x] banner is designed and content is created to indicate the above condition (design)
[x] the mechanism to enable/disable this banner is determined (product)
[ ] create a dev ticket, if necessary.

Additional context

Questions:

should the banner be automatically displayed based on certain predefined conditions?
otherwise, what would be the mechanism to enable/disable this banner?

NOTE: This is a design/dev-labeled ticket due to it's relatively small size and is ideal for collaborative work. Link to Banners Folder

Links to other issues

Relates to cisagov/manage.get.gov#1994 (email generator ticket, which is blocked for now but has useful information) Relates to: https://github.com/cisagov/get.gov/issues/288 (reduce email "epic")

cisagov / get.gov