Transcripts fix

[Dananji]

In this PR;

• Fix delay in transcript loading when switching between canvases
• Cache transcript data in Transcript component state to avoid delays when switching between transcripts and canvases (content in the transcript file is requested for initial render for each transcript, and then cached in the state for any re-renders. State is cleared when transcript is unmounted)

[cjcolvar]

This looks good as far as I can tell. I had to learn some React magic to understand what appeared to be duplicate useEffect declarations but I think I understand it now. There was one method that I wasn't able to fully grasp. It would be nice if that was made clearer but technically there isn't any reason to block this PR so I'm going ahead and approving it.