Merge in features from abandoned "Maintenance" plugin

[engram-design]

See https://github.com/carlcs/craft-maintenance which is likely abandoned

• Display a “Maintenance in progress” overlay in the Control Panel when the site is undergoing scheduled maintenance
• Announce upcoming maintenance on the dashboard and with notification banners
• Leave your users (maintenance related) messages on the dashboard widget
• Access maintenance related info from your templates using the provided template variables