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. 
Fallback logic is implemented by the XBlock function used to create the template context.