Create frontend app for YouTube video transcript annotation

[robertknight]

As part of hypothesis/product-backlog#1433 we need somewhere to host the web app that contains the video player, transcript and client. See hypothesis/product-backlog#1433 for links to design docs and UI mocks.

This will likely use the same tech stack as the frontend mini-apps in the LMS app.

• Add basic infrastructure for video player app (Add frontend infrastructure for video player app #905, Add frontend test infrastructure for video player app #907, Set up ESLint #908, Add CSS / Tailwind configuration and build process #909)
• Add transcript display, which indicates current video segment and scrolls as video plays (Improve transcript display and sync with current video timestamp #920)
• Add button to sync transcript display with current video position (Add Sync button for scrolling transcript to current segment #922)
• Add search box to transcript which filters segments to those matching search terms (Add initial version of search bar to video player #929)
• Exclude timestamps from captured selections (Exclude timestamps from selected text in transcripts #931)
• Display client sidebar side-by-side with transcript when the sidebar is open (Show sidebar alongside transcript #954)
• Fix selections spanning multiple segments in Safari (see Improve transcript display and sync with current video timestamp #920 (comment))
• Ensure TextQuoteSelector selectors in annotations don't capture text from outside the transcript (eg. from text in controls). This might be part of Constrain annotation to transcript in video player #940.
• Prevent users from annotating outside of the transcript in the video player? (ie. don't show the Annotate/Highlight controls if the user selects text in the controls UI etc.) (Constrain annotation to transcript in video player #940)
• Add keyboard shortcuts for player (Add keyboard shortcuts for video player #939)

[robertknight]

The basic infrastructure has been added in #905, #907, #908 and #909.

[robertknight]

I'm going to close this issue now as "generally done", and leave the remaining polish for other issues.