Feature/sections identifier

Contains the following:
1. Check for resource_link_id match with current session
2. URL changes for the above
3. Check section's course id for match with current session
4. Error handling for 1 and 3 above
 Note: unit tests (new and fixes for ones broken by above changes) to follow in another pull request
In this particular case, there isn't a wider try-catch block in the method. It might make sense to move the try from line 488 to 390 and remove some of the smaller try catch blocks we have (if they aren't handling special cases), including the one around check_resource_link_id , and add further exception handling (like KeyError from line 402) in the larger try block.