Avoid caching issues using last modified time

[rdwebdesign]

What does this PR aim to accomplish?

Previous code was using a similar technique, but it was using the same value (last update time of /admin/scripts/pi-hole/php/header.php ) for every file.

How does this PR accomplish the above?

A new function was created to verify the last update time of each file.

Link documentation PRs if any are needed to support this PR:**

none

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[yubiuser]

It's not working for the *.css files included in the style of

<link rel="<?php echo fileversion('stylesheet" href="style/vendor/bootstrap-toggle.min.css'); ?>">

but working for inclusions like <link rel="stylesheet" href="<?php echo fileversion('style/pi-hole.css'); ?>">

[rdwebdesign]

Yeah... because I made a Copy/Paste mistake.
Fixed.

[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-v5-20-1-web-v5-18-1-and-core-v5-15-released/60495/1