Use constants in place of class constants

[rdwebdesign]

What does this PR aim to accomplish?:

Fix #2340
Avoid the PHP Parse error when running a version < PHP 7.1.

How does this PR accomplish the above?:

The previous code was declaring constants inside a class, but this class did nothing and its only purpose was to group some constants.
The new code simply use constants, in the global scope.

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

none

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[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-v5-18-web-v5-15-and-core-v5-12-1-released/57894/1

[pralor-bot]

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

https://discourse.pi-hole.net/t/cannot-see-dashboard-screens-after-recent-pi-hole-update/58039/8