Use exercise titles in tabular coach report, add [no groups] indicator for group selector navbar in coach reports.

These are fixes for issues #374 and #466.

Two very small template fixes that immediately fix the identified UX issues.

High confidence that this breaks nothing and is ready to merge.

Also reverted a change to the student user name list table that had removed the +1 from the cell height, this now appears to be necessary.

@wangguan59 can you check that this adequately resolves the UX issues identified?