Admin setting -- Lock EmDash dashboard to read-only

[kodster28]

As you start to get more contributors inside your EmDash instance, you might have too many folks to easily communicate changes to.

This could become an issue in particular if your database is getting overloaded, where you might want to temporarily block publishing to preserve whatever caching you have in place (and prevent that load from going to the overloaded DB).

Ideal to include a Read-only dashboard toggle or something in the EmDash interface to allow this control for Admin roles.

Additionally, would be good for maintenance mode, preparing for site updates, etc. GitHub has a similar-ish functionality where you can lock a production branch by setting it to read only via the admin portal. Just helps programmatically help everyone stay on the same page during changes (expected or unexpected).

[MA2153]

Sounds good. Worth considering if we also want to stop access completely in some cases, e.g. unoptimized queries in the admin.