Coach report tweaks

Fixes #336, #2042, and #2043.

Summary of changes:
* Adds fixed axis range to scatter coach report.
* Substitutes username for first and last name if neither are present when generating user data for reports.
* On timeline report only shows student data for which more than one time point is available
* If no students are available with more than one time point do not generate a plot
* Allow "Ungrouped" as an option for display in all coach reports.
* Link to "Ungrouped" coach report from control panel.
Haha, unnerving variable name. Though you didn't introduce it here.

But wait... are you implicitly using the fact that the temporary list comprehension variable leaks out and is available afterwards?