server: redirect Grafana + Prometheus output to seperate logfile

Created by: slimsag

This redirects Grafana and Prometheus log output to a seperate logfile, located at (host / container):

• ~/.sourcegraph/data/grafana.log / /var/opt/sourcegraph/grafana.log
• ~/.sourcegraph/data/prometheus.log / /var/opt/sourcegraph/prometheus.log

The main benefit of doing this is that we separate auxiliary service logs (which are verbose, and won't usually be helpful to us/customers) from app logs (which customers should look at / inspect, etc.)

Fixes #5221

Test plan: