Handle "next" query string param in CustomLogoutView

[gumaerc]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/6789

Description (What does it do?)

This PR sets up the CustomLogoutView's _keycloak_logout_url function to properly pass the next query string param to Keycloak as post_logout_redirect_uri if it is passed along in the initial request to /logout. In this case, it would be sent instead of the value of settings.LOGOUT_REDIRECT_URL.

How can this be tested?

• Spin up mit-learn locally on this branch
• Log in to the site at http://localhost:8062/ in your usual way
• Click the User Menu in the upper right, then right click on "Log Out" and click "Copy link address"
• Open a new tab and paste in the URL, but don't go to it yet
• At the end of the URL, add: `?next=https://www.google.com
• Verify that when you hit enter, you are brought to Google
• Navigate back to your instance of MIT Learn and verify that you are logged out

[rhysyngsun]

You can use social auth's redirect sanitization for this: https://github.com/python-social-auth/social-core/blob/master/social_core/actions.py#L27-L31. You'll need to pull the value of allowed redirects more directly from the settings though.