Write a dropboxReducer

This will manage the state of our relationship with Dropbox. See the "flow" section of the design docs for how it should work: https://github.com/captbaritone/redux-dropbox-middleware/blob/master/design_notes.md

Start with the flow section of design_notes.md linked in the issue and derive the Dropbox relationship states and transitions described there. Implement the dropboxReducer so it manages that state as specified, with completion confirmed by matching the documented flow.

Written by the indexing model from the issue text.