maintenance mode

add a maintenance mode to donate-neo. we have that in donate-static right now (e.g. donate-static@1fe0f6fa) that:

• adds a banner warning about the maintenance
• refuses to submit the form

We should do something similar but not enforce it at the Javascript level, but deeper down. We could also just replace the entire page by a maintenance message, and this could therefore be done at the webserver level as well.