Intercept possible 500 errors when site maintenance is in progress and display a friendly page.
During system maintenance some pages may produce errors due to eg. database upgrades. A blanket rule would take down the entire site, which is naturally undesirable. maintenance_in_progress only displays a maintenance message when a 500 error is encountered during the maintenance window.
Admin has a Maintenance In Progress Preferences link where you can choose to indicate maintenance is in progress either by a flag in the database or by creating a file on the filesystem.
TODO: Figure out how to actually get changelog content.
Changelog content for this version goes here.