Improve UI for users in a Geo node

We don't give any visual hint for users that he is using Gitlab Geo (#76) or he is in a secondary node (read-only).

We should help guide users from a secondary node, back to primary node whenever they need to make any change. It's also important to give information to system administrations to help them diagnose configuration issues regarding Geo setup. Currently there is no way to tell if node configuration is right and replication is working, except for doing manual test.

Proposal

• Inform user when they first login to a secondary node, that he is in a read-only environment displaying this flash message: You are in a Geo secondary node (read-only). To make any change you must visit the primary node. (primary node should be a link)
• Add a little globe icon on the top-right bar of the screen with tooltip: go to primary node and link back to there
• In admin screen always display the same flash message as first login
• In admin overview display in Features, Geo with enabled or disabled state
• In admin overview display in Components, Geo node with primary or secondary for current node information.

cc @JobV @DouweM

Checklist

• Flash message on first login
• Flash message on admin screen
• Globe icon indicator on top-right bar
• Geo information in overview's features
• Geo information in overview's components

mentioned in issue #76

mentioned in merge request !451 (merged)

Marked the task Flash message on first login as completed

Marked the task Flash message on admin screen as completed

Marked the task Globe icon indicator on top-right bar as completed

Marked the task Geo information in overview's features as completed

Marked the task Geo information in overview's components as completed

Status changed to closed by merge request !451 (merged)

mentioned in commit 15191864

Mentioned in issue #76