Dropout Helper per Kurt Thiemann

This is a simple extension adds a number of small QOL features for Dropout. Additionally, it includes a watch party feature that allows for easily watching videos in sync with other people.

Currently available features:
- Save current playback rate settings for other videos
- Stop videos from starting automatically in Firefox until you allow autoplay on watch.dropout.tv
- Watch Parties: Synchronize your friends video players with yours to watch content perfectly in sync
- Scheduled watch sessions: Synchronize everyone's video players based on a fixed starting time: Useful for large scheduled watch parties, e.g. on community Discord servers

Note that volume and subtitle settings are now saved correctly by Dropout/Vimeo, and no longer require this add-on.