Maintenance mode should return some other HTTP code that 200, for example 503

[borisgrigorov]

Summary

When maintenance mode is enabled for certain resource, the maintenance screen is returned with HTTP code 200, which doesn't correspond to the actual state of the resource.

Motivation

For browser apps, it's not a big deal. But for APIs hosted through is HTTP code the simplest way to indicate the state of the resource - in this case, if it's working and when not, what is the reason for that.

Proposed Solution

When resource has maintenance mode turned on, the page should be returned with HTTP code 503.

Alternatives Considered

No response

Additional Context

No response