luci-base: ensure that base url is emitted with trailing slash

luci-base: ensure that base url is emitted with trailing slash

Now that we don't have an url token anymore, '/cgi-bin/luci' becomes a valid
url while cookies are restricted to only '/cgi-bin/luci/' and below.

In order to ensure that the first request after login refers to a path
covered by the authentication cookie, change build_url() to always append
a trailing slash if we're referring to the base url.

This should fix the login problems mentioned in #516.

While we're touching the dispatcher, also remove remaining url token code.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>