Coach report tweaks

Fixes #336, resolves #2042, and closes #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.