Maintenance Mode - Hook up API

What does this MR do?

Closes #212702 (closed)

This change is behind a Feature Flag :maintenance_mode

This change hooks up the API to the Maintenance Mode UI. The change was modeled after the approach taken for Geo Settings.

The API is managed in Vuex actions that handle the loading state as well as the data. Everything is done through the Application Settings API.

Screenshots (strongly suggested)

w/License	w/o License

	header
Loading State
Maintenance Off
Maintenance On

GIF

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #212702 (closed)