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
This gets to the question I was -- inelegantly -- trying to ask the other day. Are we going to make it standard practice to identify all identifiers in urls as kwargs, as we have done here? If so, then position in the url won't matter -- as long as we address them in the associated method as kwargs and not positional arguments. As of now, we're treating all as positional in views.py, though we could also treat all as kwargs or some one way, some the other. Not sure what the best practice is, if there is one. Probably overthinking a six of one half a dozen of the other situation. At any rate, the original plan was to have resource_link_id as leftmost anyway, so I'll rearrange.