Make the System header and footer configurable from a config file

Problem to solve

Make the system header and footer (message and font/banner colors) configurable from a config file

Intended users

Some environments where gitlab is hosted require a mandatory banner for all pages

Further details

Being able to set it from a config file allows you to set this from launch and not have to manually config from the appearance menu in the admin area. I looked for a way to set it from the host itself but I couldn't find any way other than from the appearance menu in the admin area through the web ui

Proposal

Add a variable to the gitlab.rb or other config file to allow the user to set the system header and footer message as well as the color