Use rev-specific URI in meetecho slides update notification

[jennifer-richards]

Describe the issue

Currently, our calls to the Meetecho slides update API include a versionless URI for the deck to be used. This caused problems during IETF-120 due to caching, causing the slide deck to be updated to the previous version instead of the new one. This also means that a particular rev specified to be used for the session will be ignored.

Update the API call to send a versioned URI instead of the versionless one. There were some issues related to the life cycle of the document / SessionPresentation that need to be considered when addressing this. Specifically, it must be possible for Meetecho to request and retrieve the new deck before the API call is made because they may fetch it immediately.

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