Add Maintenance mode application settings fields

Problem to solve

We want to add a read-only maintenance mode so sysadmins can block writes. In particular, this allows Geo to finish replicating before planned failovers.

Proposal

The feature flagged frontend design is shown in #211672 (closed)

• Add e.g. maintenance_mode_enabled and maintenance_mode_message to application_settings (and modify the model, set up defaults, API, etc, whatever boilerplate is needed to add setting fields, I would look at MRs of recently added settings)

Later, the frontend can be hooked up to toggle it in #212702 (closed).

After that, it'll be easy to build out the features of what the setting does.