Save video to user's device after recording

When a user records a video in the app, we should save a copy of the video to the user's device before uploading it so that if the video upload fails, the video is not lost entirely.

• Given a user is on the mobile app,
• and the user hits the "+" button to display the activity post composer,
• and the user records a video using the interface in the composer,
• when the user stops the video recording,
• the the user is presented with the composer view with the recorded video embedded,
• and a copy of the video is saved locally to the user's device.