Attempt to make V2 meeting and webinar APIs consistent for start_time

[pjkundert]

Providing a start_time for Webinar's and Meeting's create and update actions is inconsistent between V1 and V2, making it difficult to write code that handles both Meetings and Webinars.

I improved the consistency, and also detect that a datatime.datetime or .date was provided before attempting to convert it to a str using util.date_to_str.