[ENG-329] [Oath Pit] Throw the Dropbox provider into the Oath Pit

[cslzchen]

Ticket

https://openscience.atlassian.net/browse/ENG-329

Purpose

Refactor the dropbox provider to work with aiohttp3.

Changes

• Converted async with self.request() as resp to resp = await self.make_request()
• Fixed unsafe usage of the dropbox-api-result header
• Added a note and TODO for proper usage of .dropbox_request() and .make_request()

Side effects

No

QA Notes

Please refer to the ticket, which should be almost (if not exactly) the same as the box provider.

Deployment Notes

No

[coveralls]

Coverage increased (+3.6%) to 52.804% when pulling a8cce501e237e96a5321423cfd341e6525a8d059 on cslzchen:feature/oathpit-dropbox into df4af5823a90b11b7607a6e82e5602f010af8422 on CenterForOpenScience:feature/oathpit.