Switch from filesystem-based Dropbox use to OAuth API

[fwenzel]

Dropbox has a Core API and a Python library to go with it: https://www.dropbox.com/developers/core/start/python

Now that the public folder is not supported anymore, we can probably just talk straight to the API, and upload the screenshot files that way, then generate and use a share URL. That'd also enable the path inside Dropbox to be completely arbitrary.

[fwenzel]

Note, for extra credit we can request a URL like:
https://www.dropbox.com/s/hmfa6o6zom5pkol/fred-glasshole.png

and replace the domain, so that it links straight to the file again:
https://dl.dropboxusercontent.com/s/hmfa6o6zom5pkol/fred-glasshole.png

The "custom domain" feature would have to maintain at least the share code, so instead of:
http://example.com/blah.png
we'll have to do:
http://example.com/s/abcd1234/blah.png.

But at least this would be future proof.

[fwenzel]

CC @jbudd83 FYI