WIP: Add status page url to status page settings

What does this MR do?

Adds a field for Status page URL to the status page settings form in Settings > Operations.

This new field represents the url of the status page FE application. If a project has a corresponding status page setup, GitLab doesn't actually know where that service lives. If we want to be able to provide links to any status page detail view, we'll need access to the base url.

Related Issue: #213913 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) - Documentation for status page has not yet merged. I'll add a follow-up once that's gone in.
• 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