Cache submissions while migrations are running

[williamjallen]

Some database migrations take several hours to complete for large systems. To prevent unnecessary gaps in submission data during migration periods, this PR expands our existing functionality for accepting submissions while the database is offline to also cache submissions while maintenance mode is enabled.

For users of Docker-based systems, maintenance mode is automatically enabled when the container starts, and is disabled when the setup/update process is complete. Administrators of bare-metal CDash systems should enable maintenance mode with the command php artisan down when upgrading, followed by php artisan up when maintenance is complete.