Create an option for instance wide settings/fix existing "global" settings page

[sau226]

Currently, per https://meta.codidact.com/posts/285103/285189#answer-285189, site settings have to be toggled on and off per community.

While this works for the majority of settings, sometimes this is less than ideal if a setting can only have one reasonable value across the network (e.g. an API key we use the same value for on all sites), or if we need to change a setting rapidly in an emergency or semi-emergency status (such as the one above).

Can we please consider adopting one or more of the below options?:

• Build out instance wide settings that can be overridden at a site level (e.g. if a feature is default on, a site can have that feature switched off).
• Allow removing an override completely (if we reset a setting back to the instance default and later change the instance default, the site setting doesn't update automatically - removal of the override would pull from the instance setting)
• Build out a "community values" page for settings (this would let you select a setting and see what the value for it is on each site and if an override).

[cellio]

There is, in fact, a place in admin tools for "global site settings", which currently doesn't contain anything (or at least not anything I can see). Ideally, I would want that to be a place to set global defaults and show which communities have overrides set, but I think the actual overrides belong with the per-community settings. But if that's hard, we can figure something out; the number of affected users is small and we can add documentation to the page.

[cellio]

See some further notes in https://github.com/codidact/qpixel/issues/792 (closed as duplicate).