Implements #304
This PR adds Github-style image upload by drag-and-dropping an image onto the markdown editor in images and PRs.
To do this, I added a new blobs field to the issue and pull lexicons, referencing the blobs to avoid them being garbage collected. This is additive, and doesn't break existing records / decoders.
I also added an upload proxy, as the client side is not allowed to upload to the PDS directly.
getBlob doesn't work on not-yet-referenced blobs, so in order for the markdown preview to work, render the blob through URL.createObjectURL.
Low quality gif of it working in the local environment:
