exercise - fix links on dash to direct to exercise taking;  treehelper - set of utility functions 

- Added dash links for taking an exercise.
- Load question result in html dataset.
- Starburst button will take practice exam at proper path.
I'm not sold this is even useful, it actually will likely incur more overhead. The operation should finish very quickly, and it's not obvious from other parts of the tree why we are now working with raw JSON, especially if we start adding additional methods to it in the future. No premature optimization!