Common configuration for maintenance page

[jpmckinney]

If a service needs to be taken offline for a longer period, we can perhaps have a common configuration for the HTTP server to serve a static page.

[ghost]

We need a maintenance page for the Redmine server as that has regular, long maintenance periods.
I have a static maintenance page including base64 encoded OCP logo from the previous non-salt Redmine server we can use.

[jpmckinney]

Sounds good!

[jpmckinney]

Add steps to the Upgrade Redmine docs to enable/disable the maintenance page: https://ocdsdeploy.readthedocs.io/en/latest/deploy/redmine.html#upgrade-redmine

Redmine will be decommissioned eventually.

[jpmckinney]

I made a “default” configuration as part of #218 . We can make another with a different title/header (or make it configurable), and then the instruction is to change the Apache site configuration during maintenance.