In-session pausing and resumption of uploads

This allows for pausing and resumption of uploads to S3, using the pause/resume support I added to `s3-upload-stream` in https://github.com/nathanpeck/s3-upload-stream/pull/25. Pausing the upload stream will then soon cause file reading to pause, and when uploading resumes, file reading will resume.

I'm also starting to merge and clean up modules and code, and cleaning up some UX. This PR only handles pause and resume *in-session*. Though `s3-upload-stream` now handles restoring uploads from a new session, attempting to support that model through permalinks was unsuccessful. I'll have to move to a localStorage based solution.

That's for a future PR! This contains working pause/resume support for uploads from within a single browser tab/session.