Updated video management, dark mode support for embeds, and more

• Better video adder allows user to more easily add videos to collections without continuously opening and closing the add video dialog.
• Simpler video pinning: We've made it easier to pin video to the homepage from anywhere. Look for the pin dialog on the channel homepage.
• Unfurl links in Slack: Now when you paste a Rewatch video link into Slack, it will automatically be expanded to show information about that video.
• Dark mode support for collection embeds makes your embeds blend in better with your app's theme.
• Clearer private video indicator on the video page makes it easier to see when a video is private and what that means for visibility.