Add metadata labels support to the backend

[rtibbles]

Summary

Description of the change(s) you made

• Adds metadata labels fields to content node
• Adds metadata labels fields to content node exports
• Adds metadata labels export to publish utility
• Adds metadata labels to REST API for reading and writing
• Allows fields to be null to prevent default setting in migration
• Makes API always return an empty dict even when null for ease of handling on the frontend

Manual verification steps performed

1. Go to /api/contentnode
2. Fill out fields for content node including new metadata for labels (using checkboxes)
3. Post data and observe the return

Screenshots (if applicable)

References

Fixes #3263
Fixes #3324

[bjester]

Looks pretty straightforward! Just a comment that I think the MetadataLabelsField serializer could be simplified

[bjester]

LGTM 💯