Migrate Video Player

[hjiangsu]

Pull Request Description

This draft PR migrates the existing river_player with Flutter's video_player. The main reason for this is to use an actively maintained video player package. Since the video_player package does not come with built-in controls, I've created new widgets to handle most, if not all of the currently available features. This includes the following settings:

• Auto fullscreen (with a caveat)
• Muting videos on load
• Looping videos
• Video autoplay
• Default playback speed

To mimic auto-full screen when the device is in portrait-locked mode, I rotate the video player by 90 degrees. This means that the video player doesn't account for the landscape orientation of the phone (this is only while the device is portrait-locked)

TODO:

• [x] Auto-hide controls when video is playing

Issue Being Fixed

Issue Number: #1442

Screenshots / Recordings

Checklist

• [ ] If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• [ ] Did you use localized strings (and added appropriate descriptions) where applicable?
• [ ] Did you add semanticLabels where applicable for accessibility?