Settings view helper that handles fallback to global automatically

[zerocrates]

We've got a few situations where we have a setting that can be set in the site, and if it's unset, or you're not in a site, there's a global setting that gets used instead.

We're generally handling that fallback manually, but we should probably have a helper that just takes a single setting name and handles the fallback and "check if in a site" logic in itself.

[zerocrates]

One recent place this popped up is the favicon feature, but it's used other places, modules, etc.