Introduce new secure setting for the base url of the icommons api since ...

...the user and pass are tied to an instance.  Convert lecture video tool and icommons_common settings block to use this new api_host setting and also move them to base since settings are identical or very across environments.  For the canvas_url part of the common block, override the canvas url per environment (defaults to canvas.icommons.harvard.edu).

NOTE: there is a corresponding change to the ansible deploy scripts that need to be coordinated with this push.  Once this code has been accepted, I plan to merge those changes in to that project before merging this into the develop branch.