Fully support EML references in Editor

[amoeba]

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The editor partially supports EML references right now by de-referencifying them: Any references are replaced with a copy of the DOM node they point to, entirely removing the reference. See @csjx 's comment in https://github.com/NCEAS/metacatui/commit/573f5a21e8091729fe6a4e75da38eecb99d56921. That commit was the direct result of https://github.com/NCEAS/metacatui/issues/295.

This issue is to improve the editor to better deal with references. There are a few facets to this:

• The editor could seamlessly deserialize and re-serialize references (round trip)
• The editor could indicate when a reference has been used in the UI and help a user update the referenced element
• The editor could provide a mechanism for the user to use references

Somewhat related is that our indexing stack doesn't support references so any changes here might depend on changes over on https://github.com/NCEAS/metacat/issues/926.