Refactor / update reducer structure (+ actions, actionTypes, selectors, etc.)

Currently not completely in line with the changes from implementing redux-forms:

Forms	Reducers
Signup	Session
Login	Profile
Request password reset
Reset password (after password reset request)
Change password (when logged in)
Change profile

There should be a global reducer that responds to client and server errors, and a top-level component that displays them if they exist.

That means that errors won't have to be handled in reducers at all. And neither does isFetching. All that has to be stored is the profile and the token.

Update tests after this as well. Should be able to significantly reduce them.