Doing away with the concept of trapdoors

Trapdoors were non-tab links that were triggered on tab-click
in order to get tabs to behave as links do when it comes to
changing the page layout (hiding/unhiding)

Seemed unnecessary before - and is probably unnecessary now.
Tabs can very well contain the information on what to hide/unhide
Any exceptions can be made within core/trigger.js itself
[teacher]: Behaviour of report-card tabs defined more sharply

Ties in with changes committed in the last commit
Merge branch 'dev'