Fix for #187 on central: show thumbnails for coach reports

Solves central server issue https://github.com/fle-internal/ka-lite-central/issues/127

Somehow the css that styles the coachreports page was still under the distributed app. Moving it over to the coachreports app static folder and changing the pointer to it seemed to fix the issue :smiley_cat: 

grep'd the code for another ref to landing-page.css but the coachreport template is the only user. Tested this in a central-server repo by changing it within the submodule and then loading up the central server page, and then ported the changes over to a pure kalite repo.