"Leave group" feature is vulnerable to CSRF

[Evgeny Kotelnikov]

The request for leaving a group is a simple GET-request to "./leave_group". This is subject to a CSRF-attack. Should fix, probably using the "check_csrf" feature of the views, similarly to other places.

[Evgeny Kotelnikov]

Add CSRF token to the "Leave group" button (fixes #43)

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