Revoke Slack should clear Browser cookies

[dracan]

When you revoke a Slack team, it clears the authtoken from where we store it (currently the system keychain, but this will probably change (#60)) - however, if you then try to add that Slack team again, when it redirects to the https://<teamname>.slack.com/oauth page, it no longer asks to login to Slack. The cookies for this domain need clearing when the user revokes.