Allow student_id to be retrieved from a (configurable) LTI parameter

[dsteinmo]

The default LTI parameter is custom_student_id, otherwise it can be set in authen_LTI.conf.

[mgage]

For the test with Canvas used: $preferred_source_of_student_id = "ext_d2l_orgdefinedid"
as the name of the LTI parameter describing the student and set it to $User.username.
This is configured in canvas as a custom parameter -- I wasn't sure how to retrieve the student name from canvas.
(I found the reference: https://canvas.instructure.com/doc/api/file.tools_variable_substitutions.html )

ext_d2l_orgdefinedid=$User.username

The new parameter is transmitted as custom_ext_d2l_orgdefinedid
New parameters are added to the debug list:
student id:
preferred_source_of_student_id:
custom_ext_d2l_orgdefinedid:

Turn on $LMSManageUserData=1;

This seems to do the right thing. It updates the student_id properly.

The interaction with canvas and LTI seems a little flakey (or perhaps I don't remember exactly
what it is supposed to do) but this modification doesn't affect the behavior other than in the
way it's supposed to.

LTI integration deserves another thorough review. (sigh)