Geo: Improve UI for users in a Geo node

This implements visual hints to help user diagnose Geo (#76) installations, and help guide users that are in a secondary node to understand that they are in a read-only instance.

Fixes gitlab-org/gitlab-ee#640

# First login flash message
![first-login_notice](/uploads/51507c12eb8f4ec29a9e58dbc4c1b716/first-login_notice.png)

# Admin screen - Dashboard information
![admin-screen](/uploads/ee1b3b50a013cd5195c7e2047497d387/admin-screen.png)

# Link to primary node 
![link_to_primary_node](/uploads/6061d64690cda83f49145f9643d0e0bd/link_to_primary_node.png)

See merge request !451