Fixed logout view bugs, refactored w/hooks

[NWRichmond]

In addition to adding some creature comforts, this PR addresses issues #2 and #3.

Key Points

• Logout View: Now once a user logs out, they are brought back to the home page, and are able to log in again by trying to access the protected routes in account/. Previously, logging out would lead to a redirect to http://localhost:8000/account/, which is a protected route. Switching the logout returnTo location from location.href to window.location.origin brings the user back to the public route after logging out, and also fixes the separate issue of being redirected to a page beyond the root of the protected routes (e.g., account/settings or account/billing). This leads to an Auth0 error unless you explicitly add all of these routes to Allowed Logout URLs - but even if you do this, a logout redirect to a protected route is probably not desirable.
• Improved Re-Login Behavior: After logging out and attempting to access a protected route again, Auth0 login was triggered, but there was a bug which kept isLoggedIn set to false following the Auth0 authentication process. Now, isLoggedIn is set to true after the user logs out and back in again.
• Easier Navigation: Logged-in users have access to the home page (unprotected route) via the account page, and when users log in and reach the callback page, they are able to navigate to the home page, account page, and logout.
• Refactored With Hooks: The SessionCheck component in gatsby-browser.js is now tiny (~5x shorter) and easy to read. This is the first time I've toed the waters with Hooks...I'm very happy with them! Note that this requires an upgrade to Gatsby 2.5.2 to get the fix for hot-reloading with Hooks (fix(gatsby): fix hot-reloading for hooks (patch hmr) gatsbyjs/gatsby#13713).
• Helpful Hints in README: Guidance about Auth0 application settings and working with Gatsby environment variables are added to the README.

[NWRichmond]

Here's a quick demo:

[jlengstorf]

Thanks so much!