Require status page url field for status page setting

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 makes the StatusPageSetting#status_page_url field required. If started with allow_nil to enable the various parts to be merged independently.

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 - There will be no user-facing changes with this work.
• [-] 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