Use content id aliases over content ids for embedded content

[Gweaton]

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Changes in this PR

This is a first attempt at updating Embedded Content-facing code to read human-readable Content ID Aliases rather than UUID Content IDs, meaning users can assign meaningful names to embedded content.

It's a large diff, and can probably be slightly better committed to help with reviewing and prevent tests from failing.

I'm sure there are some improvements we can make to this, but as we're potentially pivoting onto other work, I'm pushing this currently working draft for future reference.

Known TODOs:

• Update EmbeddedContentSpec to use ContentIDAliases
• In tests, be consistent about whether we're using Edition.content_id or edition.document.content_id
• Do we need to update Presenters::Queries::EmbeddedContentPresenter ?
• Would tests be improved/simplified by adding a transient factory parameter to an Edition to create a content_id_alias without us having to specify one everywhere?
• Add an integration test for creating a new ContentIDAlias when PUTting a contact.

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