Add sign in button to 404 page

It would be really nice to show a friendly error if the room exists, but you have to be logged in to access it.

This applies when the room access is limited to the organization members.

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Relevant code:

• Template: https://gitlab.com/gitlab-org/gitter/webapp/blob/14399e4939c59626149b2d8ba4fdab0863cf7175/public/templates/404.hbs
• Data for template: https://gitlab.com/gitlab-org/gitter/webapp/blob/e055aa5ca88a2c2644dd0c94d0d98bcb337121c0/server/web/middlewares/express-error-handler.js#L77