Approving proposed slides does not call the meetecho API

[jennifer-richards]

Describe the issue

When slides are uploaded during (or shortly before/after the scheduled time of) a session, the datatracker makes API calls to notify MeetEcho about the slide updates. However, if slides are proposed and then accepted, the API calls are not made.

The MeetEcho API should be called when the SessionPresentation is created upon approval of the slides.

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