Allow Global Access setting

[jaredatch]

There is a very good chance I might not be understanding this setting, but if it really does only pertain to multisite setups, can we do an is_multisite() (or similar) check and only show it if that is the case?

Only thing is since that setting is passed in a giant array, obviously we can't just wrap that portion in a multisite check, we'd have to get a bit more clever.

Thoughts?

It's not just for multisite, but also for multi-network or shared user tables.

I'll need to rethink the approach, so that we can hide it only when we need to.

(In [4234]) Settings:

• Tweak verbiage to make it more clean what each setting does.
• Fixes confusion with "Global Access" and how it works.
• Fixes #1904.