POC: Maintenance mode

The purpose of this POC is to establish the following:

1. That we can use the ReadOnly Middleware to block write operations in read only mode on a primary
2. That we can toggle between read/write and read only

Proposal

• Refactor Geo related readonly code to check for a new Gitlab::Geo.readonly? instead of Gitlab::Geo.secondary?. This method will be true either when readonly: true in gitlab.yml or when Gitlab::Geo.secondary?

This POC should not take longer than 3-5 days to create. We are mainly interested in workflowsolution validation. Following this, we can break down the maintenance mode epic further to enable a first MVC.