[6.x] Separate globals config and content

[duncanmcclean]

#8343 aimed to split up global sets and global repositories into separate repositories, to bring it inline with other features where config and content is separate (eg. forms and form submissions).

However, even after it was merged, there were still a few places where they weren't totally separate, like they should be.

This PR aims to resolve these issues, as well as making various improvements to how globals work under the hood.

Changes:

• On single-site installs, variables are no longer stored in the global set's YAML file, they're now stored separately like they would be in a multi-site.
  • This allows the config and content to be properly separated, closing Please separate globals config and content ideas#1177.
  • The UpdateGlobalVariables update script will migrate global variables to the new format when updating.
• The sites a global set is localized into is now determined by the sites array in the global set's config file, rather than being determined by the existence of global variables in sites.
• GlobalSet::in() now returns a Variables instance, even when the site doesn't have any variables.
  • Resolves Globals not editable if entry is missing in database eloquent-driver#303.
• The GlobalSet::addLocalization() / GlobalSet::removeLocalization() methods have been removed, in favour of calling ->save() and ->delete() on the Variables object.
• Previously, when saving global variables in the CP, the entire global set would be saved, causing the GlobalSetCreated/GlobalSetSaved events to be dispatched.
  • If you were listening to these events to pick up changes to global variables, you should instead listen to GlobalVariablesCreated / GlobalVariablesSaved .
  • We've reflected this change internally - the variable events now trigger static caching invalidation and Git commits.
  • Fixes No way to determine which site a global was updated in with GlobalVariableSaved event #10888
  • Related: [4.x] Fix globals save event #8564

Docs: statamic/docs#1643
Eloquent Driver PR: statamic/eloquent-driver#411