According to the stackoverflow answer below, it is recommended to not
include a trailing / in cookies' path. By removing the trailing / for
our cookies path value, people's browsers visiting grafana will pass the
cookie not only to /grafana/ sub paths but also to /grafana sub paths.

This commit avoids the situation where a user would visit
http://localhost/grafana, get redirected to
http://localhost/grafana/login, and following login get redirected back
to http://localhost/grafana, but since the grafana_session cookie isn't
passed along get redirected back once more to
http://localhost/grafana/login.

ref: https://stackoverflow.com/questions/36131023/setting-a-slash-on-cookie-path/53784228#53784228
ref: https://tools.ietf.org/html/rfc6265#section-5.1.4