CheckIt import into Brightspace (D2L)

[StevenClontz]

When a problem database is generated from CheckIt to import into Brightspace, the LaTeX code for $\middle|$ does not properly render.

It shows up with the actual "\middle|". This appears in AT3 problems for image and kernel, but there are other places that \middle| also shows up, maybe back in LE4. Not sure of the fix.

TeamBasedInquiryLearning/linear-algebra#193

[StevenClontz]

Actually this is a limitation of https://github.com/roniemartinez/latex2mathml/blob/master/latex2mathml/commands.py#L23 imported by core CheckIt: it supports \left and \right but not \middle. So I'll need to figure out how to make that contribution well-upstream before it'll get fixed here. :-(