Allow users to specify frontend url when configuring status page

Context

We want to automatically create a link to an incident detail page from a frontend status page app in published issues.

Currently, the GitLab project for which a status page app is configured does not have access to the URL of the status page app.

In order to enable this, we need to allow users to supply the status page FE url to the GitLab project.

Proposal

Update the form under project > Settings > Operations > Status page to include a new text field.

The new field should be required.