Add quick-edit modal for referenced documents in Reference(s) fields

[stevenle]

Motivation

• Provide an inline quick-edit experience for documents referenced by ReferenceField and ReferencesField so editors can edit referenced docs without leaving the current page.

Description

• Add a new ReferenceFieldEditorModal component and stylesheet to load a referenced doc into an in-memory draft controller and allow saving updates back to the draft Firestore doc while updating local cache.
• Wire quick-edit state into ReferenceField and ReferencesField, passing a quickEditDocId to the modal and opening it when a reference preview/card is clicked with a normal left-click (modifier keys still open the doc in a new tab).
• Update ReferenceField.Preview and ReferenceField.DocCard to accept and call an onOpenQuickEdit callback, and prevent default link navigation when opening the modal; add rel="noopener noreferrer" to external links.
• Wrap DocPreviewCard in a clickable container for ReferencesField and add a small CSS tweak (ReferencesField__card__previewCard) to restore previous padding; also add a trailing newline in ReferenceField.css.
• Make InMemoryDraftDocController constructor accept rootKey: string | null and treat null as using the supplied value as-is, which enables using the controller with a root object like fields.

Testing

• Ran the project's automated test suite with yarn test and the tests passed.
• Performed a local build with yarn build which completed successfully.
• Ran lint/type checks with yarn lint/tsc and they passed.

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