Prevent creating non-document resources at root via import

[blms]

• Text Import can create non-document resources
• IIIF Import seemed to create a buggy document on check-in, needs further investigation

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Testing notes

In the new FairCopy dev build, 1.3.0-dev.3:

• Open a project. Instead of "Import Text" the button should now say "Import TEI"
• When you open the dialog, plaintext should not be mentioned, and it should have an additional rule that a TEI Header element must be present to import XML.
• Click "Select Files". It should limit you to just picking files with the .xml extension.
• If you import an XML file without a TEI header, the popup should tell you that it needs one to be imported. (if you need one of these, you can get one by going to a TEI document, and exporting a Text resource from inside of it)
• If you import an XML file with a TEI header, it should import successfully as a new TEI document at the project root. (If you need one of these, you can get one by exporting a TEI document from the project root)
• Blocked until Lorin is done with check-in fixes: you should be able to check in the documents.
• Inside of a TEI document, the button should still say "Import Text". From there you should be able to import plain text, and it should create a new Text resource in the document (or source document if you chose that in the dialog)
• Try importing a IIIF manifest like https://media.nga.gov/public/manifests/nga_highlights.json. It should create a TEI document with a header and facsimile element.