Add user version status endpoint and validate user version withdraw changes

[justinlittman]

For a particular user version, the status should provide:

• Is the version withdrawn?
• Can the version be withdrawn?
• Can the version be restored?
• Is this the head user version?

Validation rules:

• The latest user version cannot be withdrawn. (This also includes when there is only a single user version.)
• Only a withdrawn user version can be restored.
• Only a non-withdrawn user version can be withdrawn.

The validation rules should also be applied when updating a User Version.