Add BANNER_MESSAGE environment variable

[Turbo87]

This PR resolves #11893 by introducing a BANNER_MESSAGE environment variable. If set, the content will be available at https://crates.io/api/v1/site_metadata, which is already checked by our frontend to show a banner if the service is in read-only mode. The third commit adjusts the frontend logic to display the banner_message content instead, if it is set.

/cc @carols10cents

[eth3lbert]

I haven't reviewed the content yet, but from the description, it seems this proposal leverages an environment variable to change the banner message. I'm not sure how much I like this, as it implies that every time a banner changes, we'd need to restart the application. I wonder if it would be more appealing to use a table accompanied by either a queue or pg_notify for this job, where an admin could just make a request to change the banner. However, this might be overkill if we only need to change the banner very rarely.

[Turbo87]

However, this might be overkill if we only need to change the banner very rarely.

yeah, I assume we would rarely use this. the only advantage would be that regular admins could set the message without needing infra admin permission, but I'd say we can probably retrofit this if we really need this. since we have an on-call rotation with pagers the security WG will be able to quickly notify us to set the message for them, if necessary :)

[carols10cents]

LGTM, this gives us the functionality we didn't have before!

I think @eth3lbert 's points are good though, for the long term-- ideally in the admin web interface of dreams, we'd have an easy way to set this on mobile while at cirque du soleil without needing to log in to heroku. But this is good enough for now!