Show loading overlay when adding/removing CNAME records as it requires a FTL restart

[yubiuser]

What does this PR aim to accomplish?:

When users use the 'Save & Apply' button on the settings page, an loading overlay is shown which stays until FTL is available again in case a restart is required.
Currently, no overlay was shown when a CNAME was added/removed, but this required a FTL restart as well. This should be signaled to the users.

How does this PR accomplish the above?:

Re-use the utils.loadingOverlay function

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm