Dan f/display name validation

This PR primarily fixes a bug in the Backbone xblock string field editor view which allowed users to enter the empty string as a valid field name, resulting in a broken UI. It more generally handles whitespace issues by trimming input before attempting to save it.

Please let me know if I'm following best practices. For example, in `updateField`, is it OK to return from the if, or would it  be better to wrap the `this.runOperationShowingMessage` in an else? How about the one-time-use `oldValue`?

@cahrens @andy-armstrong