Add a "Database Cluster Status" console in Config

Summary: Ref T4571. The configuration option still doesn't do anything, but add a status panel for basic setup monitoring.

Test Plan:
Here's what a good version looks like:

{F1212291}

Also faked most of the errors it can detect and got helpful diagnostic messages like this:

{F1212292}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15667

src/docs/user/cluster/cluster_databases.diviner

@@ -65,7 +65,11 @@ effect, then continue to "Monitoring and Testing" to verify the configuration.
 Monitoring and Testing
 ======================
 
-TODO: Write this part.
+You can monitor replicas in {nav Config > Cluster Databases}. This interface
+shows you a quick overview of replicas and their health, and can detect some
+common issues with replication.
+
+TODO: Write more stuff here.
 
 Degradation to Read-Only Mode
 =============================