SharePoint media source auths on init
We probably shouldn't auth when the source is being initialised. We should only do this if we need to when SharePoint is being accessed. I know I probably thought it was more efficient to get auth done and out of the way BEFORE we moved on to anything else, but the source CAN be initialised and not used (see #240) and if auth then crashes it can break everything, even though we don't need to actually access SharePoint at all.