Using utils.showAlert for Local DNS and Local CNAME pages.

[rdwebdesign]

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What does this PR aim to accomplish?:

Fix #1357.

How does this PR accomplish the above?:

Replacing individual local functions with the global utils.showAlert function.

What documentation changes (if any) are needed to support this PR?:

none.

[yubiuser]

I've tried it but I never get the success message (but action is performed successfully).

Additionally, (even if not the main concern of this PR) it would be nice if the input fields were cleaned after action was performed.

[yubiuser]

I'm not sure if the fields should be cleaned after something is deleted. A use case would be adding in data already, but spotting something to delete first, which would trigger the cleaning

[yubiuser]

The missing success message was due to a FTL resolver startup delay configured by DELAY_STARTUP=5. After removing the delay the success message appeared as expected.
This behavior gave the idea to pi-hole/FTL#1349

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-16-web-v5-13-and-core-v5-11-1-released/56384/1