Enable un-authenticated handler urls

Updates to depend on the latest version of XBlock, which includes
support for service-to-service (thirdparty) handler urls, which aren't
authenticated with a user (unlike handler requests coming from the
xblock client-side javascript).

Co-author: Ned Batchelder <ned@edx.org>
Rename the LTIModuleDescriptor to LTIDescriptor to follow convention
Add a per-course anonymous student id

This does not yet replace the existing per-student anonymous id, but
is intended to do so in the future.

Co-author: Alexander Kryklia <kryklia@edx.org>
Co-author: Ned Batchelder <ned@edx.org>
Co-author: Oleg Marchev <oleg@edx.org>
Co-author: Valera Rozuvan <valera@edx.org>
Co-author: polesye
Make FieldDataCache use the user from scope_ids, rather than its pre-configured user

Co-author: Ned Batchelder <ned@edx.org>
Add grading functionality to LTI xmodule

Co-author: Alexander Kryklia <kryklia@edx.org>
Co-author: Ned Batchelder <ned@edx.org>
Co-author: Oleg Marchev <oleg@edx.org>
Co-author: Valera Rozuvan <valera@edx.org>
Co-author: polesye
[BLD-384]
Merge pull request #1791 from cpennington/rebased-lti-graded

BLD-384: Add grading to LTI module