v.18.07.00: Individual message improvements / migration to JSON (#3)

* Added test targets, fixed crash when count > defined gradeables

* Fixed percentages. Not sure how a 0 point, all EC category could work,
despite existing comment about it, so just ignoring that problem for now.

* Further percentage fixes

* First draft of using only the message.json instead of message.html

* special_message and exam_seating can work together