Clean up the unused version_authors.user_id column

[smarnach]

We can consider removing it from the database schema.

[carols10cents]

Yep, looks like we talked in this PR's comments about doing that but never did 😂

Implementation instructions:

• Create a diesel migration by running diesel migration generate and make it drop the user_id column on the version_authors table. Nothing has ever inserted any values into this table (I checked in production and all rows have NULL)
• Commit the schema changes made by running the migration, which should include removing these lines and this line
• Remove this line in the database dump config
• Remove the empty users key from the /authors JSON. This has never returned any values, so I highly doubt anyone will break because of this, they should be ignoring this field.
• Fix any compiler or test errors