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
The key error try catch (line 61) was already there and was not changed for this iteration. Leaving as is.