Allow Users to Choose (override) Video Thumbnails

Right now Gallery selects the video thumbnail automatically by capturing the frame 5% into the duration of the video. While this works in most cases it isn't ideal for every case, and allowing for users to choose their own thumbnail for videos isn't too difficult of a feat. Especially if we still auto-generate a thumbnail on upload, and then give users the ability to select a thumbnail.

Since Gallery uses the HTML5 Video Element this feature can take use of that API. It could be as simple as providing a button on the video playback page that would take the current playback position of the video and then use that frame for the newly generated thumbnail.

On the backend this would probably include downloading the video from S3 and then running the thumbnail generation function. We'll need to change that function so we can plumb the playback position to it somehow, but that can be left up as an implementation detail.

首先定位 issue 中描述的视频播放页面、上传流程和缩略图生成函数。跟踪当前 5% 时长处的缩略图是如何创建的，以及视频是如何存储在 S3 中的。完成的标准是用户可以选择当前播放帧作为缩略图，同时上传时的自动缩略图生成仍然正常工作。