Better Cloudinary authentication

Current state
- users have to be logged in to the same browser, but it's not always possible to log in via the Cloudinary widget
- experience is not consistent across browsers

Ideal state
- users could log in to Cloudinary without leaving the Decap app
- switching Decap & Cloudinary instances should be easy
- a consistent experience across browsers

Possible solution
We would have to look into the docs for the Cloudinary media library widget - the auth section: https://cloudinary.com/documentation/media_library_widget.

Start with the Cloudinary Media Library Widget documentation, especially its authentication section, as linked in the issue. Define the integration approach around in-app login, easy switching between Decap and Cloudinary instances, and consistent behavior across browsers; the work is done when those goals are met.

Written by the indexing model from the issue text.