Fixed 404 error for robots.txt

fix(settings): reverted prev settings fix for detecting public_gen folder in dev, caused issue for prod build

Fix dataproxy_test.go tests

This change fix dataproxy_test.go tests that was failing with:

pkg/api/dataproxy_test.go:17: not enough arguments in call to NewReverseProxy
pkg/api/dataproxy_test.go:39: not enough arguments in call to NewReverseProxy
FAIL	_/home/ubuntu/grafana/pkg/api [build failed]

tech(): made config system check for generated css or javascript files and panic if there are none, also if there is a public_gen directory it will use that, even if static root is set to public

Update docstring for postgres session provider

The postgres provider is named postgres and not postgresql. For somebody
configuring the server from the config file example, it is very easy to
write an invalid value into the file and accidentally use the "memory"
provider instead because of a typo.