Moves the get_demo_organization_deadline_days_remaining helper from
"web/src/navbar_alerts.ts" to "web/src/demo_organizations_ui.ts".
Prep commit for updating the navbar alert for demo organization to
have a button that opens the modal for converting the organization
to a permanent organization.
As part of the banner redesign project, this commit applies the new
banner styles to the desktop notifications navbar banner.
This commit also acts as a base for the following banner redesign
commits by defining the `banners.ts` module, which now acts as the
centralized module to handle the banner rendering and sets up the
common event listeners.
