Add MOTD option for Source Interface

[zenmonkeykstop]

Description

(We discussed this before, but I can't find an issue...)
Add an admin-configurable message widget on the Source Interface homepage, to allow for motd-stlye notifications or descriptions of the site's purpose

How will this impact SecureDrop users?

Admins could inform users of upcoming outages, and could use about messages to help sources decide if the instance or SecureDrop in general is right for them.

How would this affect SecureDrop's threat model?

Messages would need to be plaintext or using a restrictive subset of html, to protect against embedded links or media that might de-anonymize users.

User Stories

As an admin, I want to be able to provide useful site-specific information to prospective sources.