Configure global banner with environment variable

[williamjallen]

It's currently possible to configure project-specific banners, as well as a global banner which is visible for all sites. The global banner is currently one of the only settings which is not configurable by environment variables. Additionally, the current implementation of the global banner means that it's not possible to use foreign-key constraints on the banner table.

Proposal: move the configuration of the global banner from an option in the banners UI to an environment variable alongside the rest of the global configuration settings.