Add an institutional preference for adding a message to login screen

[maxpatiiuk]

Blocked by #3169

Max: the login screen for their demo instance includes a message with login credentials - https://demo.mathesar.org/auth/login/
Should we do the same? (allow admins to set a login screen message in institutional preferences)

Grant: Yes!!! For demo fish it would be so useful

[grantfitzsimmons]

Close as we set up a guest user

[maxpatiiuk]

While the issue is resolved for the demo server, I belive this could come in handy for many institutions.
Sysadmins could post a notice of upcoming maintenance/downtime, or a link to a policy page, or information about how to sign in/whom to reach to get help.
Many use cases...

Could be reduced in need if we let admins send notifications, or let them customize the welcome page even more than they can now. Still, putting a notice on the login page is the simplest solution