Add config option for post logout redirect url

[guerler]

Currently our post log out url is fixed to /root/login?is_login_redirect=true, and this constant is used in the client and the backend. This PR suggests to add the post logout url to the Galaxy configuration and handle it consistently in both cases. It also optionally enables admins to customize the logout urls.

Additionally this PR contains a fix to properly route the context-menu new tab handling.

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