Prevent command injection via admin email

[AzureMarker]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
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• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)
• I have Signed Off all commits. (git commit --signoff)

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What does this PR aim to accomplish?:
Prevent a possible command injection via the admin email. Credit goes to Christos Pierris (@pr0tean) for finding the bug.

How does this PR accomplish the above?:
Perform extra validation on the input email.

What documentation changes (if any) are needed to support this PR?:
None

[DL6ER]

Following characters are preceded by a backslash: &#;|*?~<>^()[]{}$, \x0A and \xFF. 'and"` are escaped only if they are not paired.

This will allow @ to pass through and this is the only thing needed here.

[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-4-3-2-release-notes/23852/1