Coach report bugs

This fixes #662 and, oh-by-the-way, #578.

Issue:
* We have multiple json files with topic data, and so we need to be careful about which one is being used where.
* In a recent coach report update, I used topic data from the knowledge map, which only contains a subset of required information.  This missing data then left blank spots on the student coach report.

Changes:
* Clean up unnecessarily complex / embedded functions for accessing knowledgemap topics, which hid the fact that knowledgemap topics were being used.
* After getting the knowledgemap topics (which are the entry points into the topic tree and reports), use only the slugs to get the full topic information, to pass around in reports.

In addition, for #578:
* Fix logic such that, if there are no groups *AND* there's a topic selected, only then give the error in #578.

Testing:
* Checked student view coach report and tabular view with only ungrouped students (and no groups created)