Globals tweaks

[jasonvarga]

Globals no longer need IDs.

• Technically they still have ids, but they alias the handle.
• IDs were introduced to let you rename the handles without breaking things. But if you change the handle you'd need to do some manually work anyway (templates, etc). The ID just made things uglier. Not enough benefit.

Better separates editing of the global set itself, and its variables.

• Think of the global set like a collection, and its variables like an entry.
• The create global set screen now uses a consistent mini-wizard view.
• The edit global set screen is now its own route/form instead of a stack while editing the variables.
• Separate globals and variables into different controllers.

Multisite behavior is more intuitive.

• Behaves more like navs. If there's a file in a site subdirectory, then it exists for that site.
• You no longer need to define which sites it should be available in on the set level. The sites array is no longer needed.
• When editing a global set, you can select which sites its available in, and which site each should use as its origin. Hitting save will create and/or delete the appropriate files.

And more

• SiteSelector.vue (the top level CP one) is renamed to GlobalSiteSelector.vue.
• The site selector used on the nav view is moved up a level, not specific to structures, and now shared by globals.
• Some other related tidying up.