Logout should be via POST, not GET

[chaoran-chen]

At the moment, the user gets logged out (or is supposed to get logged out (#308)) when navigation to /logout via GET. This is not a good behavior as a GET request should not modify a state. An action like logout should only be performed via POST. I.e., instead of the current <a> link, we should have something like:

<form method="POST" action="/logout">
	<button type="submit">Logout</button>
</form>

And the page should only perform the logout, if it is called via POST.