Add editing view for Open Assessment block in Studio

* XML serialization/deserialization
* Studio view supporting read and update operations
* Validation of input XML, including some semantic rules for assessment fields.

This duplicates some functionality in scenario parser.  I wanted to rework the XML deserialization code for two reasons: (a) we're now accepting input from users, who we don't necessarily trust, so we need to be much more careful about how we handle it, and (b) I wanted to make sure the file format more closely aligned with the model definitions in the peer app.  I plan to replace scenario parser with the new XML serialization/deserialization methods in a future PR.

I wanted to get this up for code review, even though it's not 100% complete.  Still missing:
1) View-level test coverage
2) Remove duplication with scenario parser
3) Styling