Update the conversation at the given index for the given organization (accessible to Secretariat or original same-organization author)

<h2>Access Control</h2> <p>User must belong to an organization with the <b>Secretariat</b> role or be the original author of the conversation in the same organization</p> <h2>Expected Behavior</h2> <p><b>Original Author:</b> Allowed to update only the message body of a conversation posted by them</p> <p><b>Secretariat:</b> Allowed to update the message body and/or visibility of any conversation</p>