Geo: Help admins diagnose configuration problems

We already show some errors on Admin > Geo > Nodes, but they are often not helpful enough.

My first recommendation is to run gitlab-rake gitlab:geo:check on primary and secondary.

Proposal

• We could instruct the admin to "Try running gitlab-rake gitlab:geo:check on each Geo node running unicorn or sidekiq". (Weight: 1)
• Add button that runs Geo checks on "this" node? (Weight: 1, if we don't mind displaying output as-is, probably in a collapsed div) (Better than that could quickly get complicated) => Opened a separate issue for this proposal https://gitlab.com/gitlab-org/gitlab-ee/issues/10374
• Automatically run Geo check on "this" node every X minutes when there is a problem? (Weight: 3, if we don't mind displaying output as-is, probably in a collapsed div)