(#19879) Add support for custom links on top bar.

Allow the user to add custom links on the global navigation bar
by editing the settings.yml file.
Example given in setttings.yml.
Merge branch '19879-add-custom-links' of https://github.com/Thibo-Abidbol/puppet-dashboard into 19879-add-custom-links

Conflicts:
	app/views/shared/_global_nav.html.haml
	config/settings.yml.example