A new join table - commentaries - between questions and tex-comments ...

Going forward, it would be become increasingly common - and necessary -
to quickly load all TexComments for a Question. Hence, the join table
Only one entry per (question, tex-comment) pair in the DB
Query Commentaries table to get applicable comments for a question
 Removed superfluous alert
No need to subtract 50px from y-offset if grading stabs ...

The 50px was subtracted to account for the shadow that was overlaid on
top of the overlay. That shadow is not there in the stabs grading panel
No need to redefine method question
No need for restricted argument in preview_images
Force IST when finding seconds_since_midnight
First draft of the stab grading panel ...

TBD:
  1. reporting an issue
  2. Storing quality in stabs/grade
Merge branch 'master' of github.com:abhinavc/web-application