Expose status page url to status page settings API

What does this MR do?

In order to automatically link to status page detail views, we need to ask the user to provide the url of their status page frontend.

This MR exposes the ability to update the field via API request. Requests including or excluding a value for the new field will be accepted, allowing BE to be separated from FE changes. Future MRs will add the FE, and then make the new status page url field fully required after all other MRs have merged.

Related MRs:

• Add field to model & DB
• Allow CRUD actions via API
• Add field to settings form in UI
• Require status page url field
• Documentation - TBD (appropriate docs have not merged for status page yet; update will come after)

Related Issue: #213913 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - No user-facing changes occur in this MR.
• [-] 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