Fix download state not being reactive in offline mode.

[dero]

Summary

Fixes #127

The VideoDownloader instances take a little time to initialize, i.e. to fetch download status from IDB. After a hard refresh, the downloader instance usually still was in an indeterminate state, which the application incorrectly interpreted as the video not being downloaded locally.

I have updated the downloaded implementation to broadcast state changes and then hooked the single video page to listen to those changes. Once the downloader is initialized, the disabled class is removed from the wrapping article element in cases when the video is available, i. e. when it's available for offline playback.

Screencast to illustrate the fix

web-dev-offline-status-fix.mp4

[felipebochehin87]

Tested against https://kinoweb-dev--staging-2ra3ji0i.web.app/

1. Opened https://kinoweb-dev--staging-2ra3ji0i.web.app/
2. Clicked on a video
3. Clicked on "Make available offline"
4. Clicked on Settings
5. Toggled on "Show offline content only"
6. Clicked on Kino icon
7. Clicked on the video that is available offline
8. Clicked on "Remove offline version"
9. Clicked on "Undo" - download worked
10. Clicked on "Make available offline" - download worked
11. Clicked on "Remove offline version"
12. Waited a few seconds - video was removed from the offline list, buttons were disabled and image background turned to gray ✔️

Kino.166.mp4