Update our media send flow to allow users to send multiple images/videos
at once. This change includes:
- New in-app media picker flow.
- Ability to caption images and videos.
- Image editing tools are made more prominent in the flow.
- Some fixes to the image editing tools.
Previously, we were always rendering images as squares. Instead of doing
that, we now render them as close to true-to-size as possible (within
reasonable min/max width/height boundaries).
