[6.x] Multisite globals selection

[jaygeorge]

Description of the Problem

On multisite globals, a long list of localizations makes setting origins one row at a time cumbersome.

Before:

(on the multisite-entry-grouping branch). Here there is no way to set multiple origins at once:

What this PR Does

Builds a better globals origin workflow on top of site grouping from multisite-entry-grouping:

After:

• Multi-select plus mass “Set origin to…”
• Grouped origin pickers
• Renames the section to Localizable (existing translation) instead of Sites (which duplicated the label)
• Prevents circular origins (e.g. Tokyo → Japanese originating from NY → English and vice versa)
• Adds tests
• Improves issues mentioned here Multisite - Globals  ideas#657

How to Reproduce

1. On a multisite install with site groups configured, open Globals → a set → ⋯ → Configure, and scroll to Localizable.
2. Select several sites and use “Set origin to…” — selected rows should get that origin and enable if needed.
3. Try to set origins in a loop (A → B and B → A) — it should error instead of hanging.
4. (Optional) Create a circular entry origin chain and try to save — it should be rejected; reading values should not hang.