Redmine

Per default, Rails uses "/" as path in session cookies. When mounting
Redmine on a relative URL root, say '/redmine', the path in the cookie
should also say "/redmine". Otherwise a browsers sendsi the cookie to
all applications running on the same host. This is problematic when
running more than one Redmine instance on one server.

Fix it by setting the cookie path to config.relative_url_root when set,
"/" otherwise. Rails automatically sets this config from the environment
variable RAILS_RELATIVE_URL_ROOT.

Related to Patch #3968