[Task] Add "killswitch" to disable any preparation or georeferencing input sitewide

[mradamcox]

Describe the task
Need to have a basic, application-wide setting that disallows user input. All interfaces should still be operational, but data submission is disallowed.

This would be useful for:

• Temporarily disable the site during maintenance or troubleshooting
• Create a public staging site to preview front-end development for feedback