Logout link ends with `//logout` instead of `/logout`

[consideRatio]

It seems that a template is provided that results in a logout link with double / in it. {{ base_url }} seems to include the /, which I recall is the standard for this configuration.

default-hub-homepage/templates/error.html

Lines 23 to 27 in 1c3d5bc

	<div class="text-center">
	<a role="button" class='btn btn-jupyter' href='{{base_url}}/logout'>
	Tap to try a different account
	</a>
	</div>

There seem to be examples from oauthenticator where logout_url is used directly, which seems more accurate if possible instead of the hardcoded /logout.