Update pages config async when updating domain

What does this MR do?

This makes the update to the pages config file async when updating a pages domain.

This also skips the update if pages aren't deployed, since in that case, there would be no config file on disk to update.

#231336 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry Change behind a feature flag
• [-] 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