Limit Upload File Sizes to GridFS.

This commit puts a limit on the size of files that
course staff can upload to MongoDB. The limit is
enforced on the frontend in javascript as well
as backend via the /upload endpoint.

The limit is hard-coded in cms/envs/common.py and
may be changed according to the user's custom
needs.

If the user tries to upload a file that's too
large, an error message will pop up, with a
customizable url that pointing the user to an
external page with an alternate upload procedure.
This url is specified im cms/envs/common.py.
If not set, this url will not be displayed.