Remove jQuery inputmask

[DL6ER]

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What does this PR aim to accomplish?:

Fix #928

This also removes a rather big library from the settings page, making the Pi-hole footprint somewhat smaller.

How does this PR accomplish the above?:

Remove the jQuery inputmask validator. Entered values are still validated in the background. Instead of preventing the user to be able to enter invalid data, we now shift the checking into the backend (where it is already happening) and display meaningful errors.

Custom 1 DNS server: 127.0.0.1#5353fff produces this error:

Custom 1 DNS server: 127.0.0#5353 produces this error:

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

None