Subshell 0.81-beta

Subshell 0.81: feed videos keep their shape

Highlights:

• Fixed: tall and very wide videos in the feed were stretched and cut off at the edge while playing.
• Mention people in comments: type @ or u/ for people from the thread, then Reddit's people search.
• Subshell checks for updates when you come back to it, so a new release shows up within the hour.
• The update dialog can turn on update notifications if Android has them switched off.

Under the hood:

• The feed video surface now starts in the video's own shape; it used to start at the frame's full width and shrink, and the platform kept the first width.