Move the cookie helpers to cmk.web.utils

language_cookie and user_frontend_settings_cookie only needed cmk.gui for the
Request and Response types. Add a ResponseProtocol next to RequestProtocol and
let the helpers take the protocols, then move them to the web package.

Drop del_language_cookie and del_user_frontend_config_cookie on the way, they
have no callers. del_language_cookie was also wrong: it deleted at path "/"
while set_language_cookie writes the cookie at url_prefix(). Without them the
protocol needs no delete_cookie member.

ResponseProtocol.set_cookie declares "path" without a default. werkzeug
defaults it to "/", which is outside the site's URL prefix, so callers have to
name the path.

CMK-37528

Change-Id: Ic08908aef0d2a99b9646dd814722c56ebaf0c835