Add status page url to setting form in UI

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 adds a new Status page URL field to the status page setting form in the UI. If a user attempts to edit their existing configuration, they will need to add a value for the field.

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

Related Issue: #213913 (closed)

Screenshots

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