Authoring: label for criteria

[ORA-657](https://openedx.atlassian.net/browse/ORA-657): Create label field for Criterion

* Update the edit rubric templates to include both a name and label for criteria and options
* Update container JS to retrieve both name and label for criteria and options
* Update schema to make 'name' optional and 'label' required; the XBlock editor handler assigns UUIDs to criteria and options that don't have names
* Assign labels to criteria/options if they do not have any
* Update the grade template to display criteria/option labels if they're provided, otherwise use the 'name' field
* Update student facing rubric templates to display the label if it's available
* Add label field to XML problem definition.
* Add label field to assessment
* Add label field to the data dump script.

I'm going to test this more thoroughly in a sandbox tomorrow.

@stephensanchez Please review. 
The label field is NOT NULL and defaults to an empty string.  There's also fallback logic in the templates to use the name if no label is defined.

This is mainly to clean up the database models so the data dump is consistent.