Sketch out middleware behavior

The middleware will listen for special actions (much like a reducer) but instead of returning a new state, it will kick off async tasks, and dispatch actions based on what it sees.

Create a switch statement with all the actions it should listen for, and add a comment saying what it will do in each case.

No file, test, entry point, or action list is named in the issue. Inspect the repository's middleware entry point and existing Redux/Dropbox integration first; done means documenting the intended actions and adding the requested switch cases with comments describing each behavior.

Written by the indexing model from the issue text.