Layout engine kaboom

[Former user]

I registered twice as a student to a course (to be able to check that labs work with automatic checks), and god the page twice. This is not a rendering issue, but there's actually an entire copy of the page in the #signup-form-row div, including <title/> and <meta/> tags.

[Evgeny Kotelnikov]

Do not show the login page to logged in users (fixes #137)

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