Minimise context exports

[jackherizsmith]

This makes it tidier and easier to keep on top of. E.g. you export setUser so that you can log the user out from <Nav> - however the logOut function you call (also from context) runs setUser(null) anyway, so you don't need to export it.

Meanwhile!

<AuthContext.Provider value={{ user, signUp, logIn, logOut, setUser }}>
  {loading ? <p>loading</p> : children}
</AuthContext.Provider>

This is awesome and works really well. What I would suggest instead is exporting loading, and using that in your <Layout> component, so that you don't lose the page each time, i.e.

<AuthContext.Provider value={{ user, signUp, logIn, logOut, loading }}>
  {children}
</AuthContext.Provider>