Update to imsglobal/lti

[battis]

Steven Vickers has released the updated, LTI 2.0-compatible version of his LTI Tool Provider framework, via composer, as imsglobal/lti and we should update (and dramatically simplify) dependencies here. HOWEVER, because the new tool is LTI 2.0, it requires reworking all of the MySQL backing tables, which means that, essentially, this would require potentially writing a conversion script or being part of a breaking update (or both).