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 try/catch pattern below is repeated many times throughout views.py.  I think it would make more sense if this was a decorator; however, as is I don't think we need to worry about catching the KeyError that may occur if the LTI_LAUNCH parameter isn't in the session.  All of the views are decorated with the lti_role_required decorator which takes care of that for us and will raise an exception.  If, somehow, that were to happen here then I think letting Django handle the 500 error would be sufficient.  So basically, I think this block can be shorted to just be what's in the `try` condition.