Canvas: 504 error when pushing grades to Canvas

[pdpinch]

Steps to reproduce

1. Find a course with 500 students, like the test course 8.01 F2020 on mitx-qa-next
2. Go to the Canvas tab of the instructor dashboard
3. click "Push all MITx grades to Canvas"

Expected Results

Grades are pushed and we get a positive response message

Actual results

We get a positive response message, from edX, but it contains a 504 error.

Note that the assignments do seem to get set up correctly, but no grades are sent.

[pdpinch]

@noisecapella I think we can close this, so I will.

[pdpinch]

Fixed by #183