Support a clean maintenance mode

[stevejalim]

We should be able to slide the Portal into maintenance mode so we can perform breaking upgrades if need be, or silo the site if there is a problem

Option 1 (ideal): DNS or load-balancer-level remap so that all requests for any page in the site return a simple HTML page which can be loaded from an S3 bucket (possibly the user-media/assets bucket). The HTML page would also need CSS and possibly image assets enabled.

Option 2 (less good): A Django-level remap where all requests for any page (other than the admin pages) instead are handled my a simple template-based maintenance-mode page. This can be enabled via env vars, so would need a deploy to enable, which may mean it's not suitable for all situations.

[stevejalim]

@limed Heya dude - how easy is Option 1 to acheive? Would it always require someone with AWS access to enable?

cc @valgrimm - this would be good for hard launch, just in case, but if not we should do it as a fast-follow feature

[valgrimm]

👍