Add option to cancel file uploads that have not completed and/or have been deleted

Not a bug, just a feature request.

(This is all via the Keybase MacOS app)

I started sharing a large file (2GB) over KeybaseFS but the time-to-land looked like it was going to be much longer than initially expected (presumably cos of the cost of encryption - fair enough), so I deleted the document and restarted the Keybase client.

After restart, the deleted file upload resumed with no obvious way to kill it, yet the destination folder is reported as having no contents.

The report concerns a 2GB upload through KeybaseFS in the macOS app that resumes after deletion and restart. Start by reproducing that sequence and locating the upload and cancellation entry point; done means users can cancel incomplete uploads and deleted-file uploads no longer resume invisibly.

Written by the indexing model from the issue text.