Display confirm modal before leaving Static Site Editor

What does this MR do?

Displays a confirmation modal when the user attempts to leave the Static Site Editor and there are unsaved changes.

CAVEAT: Currently, The Rich Content Editor removes the last empty line of a page’s source automatically. This is a bug in Toast UI and causes the editor to detect "changes" as soon as the editor is initialized.

Screenshots

Showing different browsers because all of them show a different message 🤷

Firefox	Chrome

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

Closes #216865 (closed)