Refactor storage to remove circular dependencies with auth

[yknl]

auth.ts and userSession.ts both have dependencies on storage. Either all storage functions should be accessed through userSession or storage functions should be separate and take a userSession object as input.

[kyranjamie]

#795

[aulneau]

I'd be open to talking about the pros and cons of the user session in general and if it's still something we want. Anyone else?

[zone117x]

I think userSession essentially represents the state of an authenticated user, and it makes sense to be defined by the auth lib. If we take that approach, the storage functions should be separated out.