TreeSets are annoying. contains() is calculated with the comparator,
which can lead to some weird bugs. Made sure the comparator didn't think
two items with the same date were identical.
Also fixed stableId generation to avoid any potential weirdness there.
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.
