Add file options to event embed modals

[camdendotlol]

Summary

This PR updates the CompareEventsModal and SingleEventModal used by the rich text editor to require the user to specify the audiovisual file when the user is trying to embed a clip on an event with multiple AV files.

The behavior is split into multiple cases (I'm not a fan of how complex this is from a dev standpoint - I worry that we might lose track of the different functionality branches in the future and this could become brittle):

• the user selected the Full Event option: no change - the entire event is embedded and we don't need to specify and AV file
• the user selected the Clip option and more than one AV file exists in the event: show a list of radio options for the user and require them to select which AV file to embed
• the user selected the Clip option and only one AV file exists in the event: do not display the radio options, but silently save the UUID of the one AV file when the user submits the modal
  • it's necessary to save the file UUID even when there's only one file for two reasons: 1) the user might add another file to the event in the future, and 2) it would be simpler for the frontend to be able to rely on the file property always existing instead of needing two branches of functionality for looking up files

Screenshots

Single Event

Event Comparison

(This modal overflowed on my laptop screen. I updated the styles so the buttom bar floats and the rest of the content is scrollable when necessary. This change will only be noticeable when a modal is too tall to fit on the screen.)

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