Redirect already logged in users who visit the /auth/ url to /membership/

[petercalvert]

- Do you want to request a feature or report a bug?
Bug

- What is the current behavior?
When a user has already logged in, but uses the 'Membership Login' link in the site footer, they're taken back to the login form, which hangs - the login button presents the content 'signing in' before adding any credentials but doesn't redirect the user. Because the button is disabled too, even after re-entering login details, the user can't progress.

- If the current behavior is a bug, please provide the steps to reproduce.
Login as normal, then visit the auth URL.

- What is the expected behavior?
Redirect the user to the /membership/ url if they are already logged in.

- Please mention your operating system version and the version of your browser
macOS 10.14 - issue replicated in Chrome 70 and Safari 12