Importer: Sanitize UUIDs

Closes #3844
- Before importing, all data is checked for incorrect UUIDs. If check
fails, data is sanitized and outfitted with correct UUIDS.
- Scaffolding allows for easy implementation of additional
sanitization, should that be required in the future.
- Test included.
Instead of keeping a list hardcoded list of tables here, I would recommend just checking `core/server/data/schema.js` to see if a UUID is required for a given table:

```
var tables = require('../schema').tables;

if (tables[tableName].uuid)
...
```

You can also use `tables[tableName].uuid.nullable` to check to see whether or not a null/empty string is allowed.