(WIP) Implement fetching transcripts using video ID

This pull request is [will be] a completed version of #43. Transcripts will now be retrieved using the transcript ID *or* the video ID, saving editors from needing to find the transcript ID.

Other changes:
* Updated the template accordingly  (this was TODO item 1 on #43).
* If an error occurs when loading the transcript, the transcript UI is not shown (this was TODO item 2 on #43).
* Removed some HTML code from the `.py` file and implemented error formatting in the template instead
* Added missing `URLError` import

I have marked this WIP because I still need to:
1. Test! Waiting on a 3Play API key which I need in order to test this.
2. Possibly may need to gracefully handle a "No transcript exists for this video" response from the API? Need API key to find out how/if that happens.