Add maintenance warning banner

Describe the feature
We have a warning banner for non-production environments. For production, we might want to add the ability to put up a maintenance warning, e.g. "There will be a scheduled maintenance window from (datetime) to (datetime), Lexbox may be unavailable during that period". Or "Lexbox is now undergoing scheduled maintenance and is read-only".

In addition to the banner at the top of each page, it would be ideal if that message could be returned to hg push requests.

The message could be injected either with an environment variable, similar to how the PUBLIC_ENV_NAME var is used, or else with a text file (no larger than 4 kB, expected size a few hundred bytes at most) whose contents would be displayed. Or else we add a button to the admin dashboard to set a maintenance message, and/or have an admin-only API to set such a message, and the message is stored in the Lexbox DB. That would allow setting and clearing the message without having to restart the server process to pick up on an environment variable change.

Also related: https://github.com/sillsdev/languageforge-lexbox/issues/1956

Who is this feature for?
Users

Pages affected
All pages, we would add the banner in AppBar.svelte in the same place that the "This is a development environment" banner appears, so it's at the top of every page.