More sophisticated system for uploading images

[borisyankov]

Sending an image message takes two steps:

• uploading the image
• sending the message with the image

Two problems with that:

• If the first step fails, the upload is never retried
• we show a message preview only after the image upload succeeds, which is the longer part of the process

Ideally, we will track all image uploads and retry them if they fail.

[gnprice]

This is a multi-issue issue, which is better tracked as separate issues. (Something like this could be useful if describing work on a specific system to address the issues, but this one never functioned as that.)

The lack of a preview or other feedback is #2684. For the retry, I'm dubious that just always retrying is the right thing anyway. #3829 describes the foundational thing I think we want here, for messages in general; and we can file specific issues for further upload-specific wrinkles as we get into them.