Migrate canvas_course_id from studio to plugin

[arslanashraf7]

Related to:
#7 (comment) & #7 (comment)

Description:
We created a field in studio advanced settings to map paltform's courses with Canvas Course IDs. (mitodl/edx-platform@b61152ac70#diff-5e5a9a90f02439d6bcd1f65bc96da133eedae87338ba248f7bd49a4752fbbaf7). Since the plugin support in openEdx doesn't currently support this type of enhancement so we couldn't move it to the plugin.

What needs to be done:

• We should attempt to migrate this part from the platform into the plugin. (Possible approach defined below)
• Migrate the existing keys from the Studio's Advanced Settings to the newly created mapping model defined below.

Possible Approaches:

1- One possible approach could be to create our own model in the plugin to map the platform's course Ids to the Canvas course Ids. This could be a simple model primarily having two keys e.g. patform_course_id, canvas_course_id probably with unique together constraints or any other if needed.

Frontend? Currently canvas_course_id key shows within Advanced Settings tab of a course in the studio. But, When moving this into the plugin based on the approach defined above, where/how should it look in the frontend? A simpler solution could be to set it in Django Admin but Is it feasible for the users to set this key in Django Admin?