Don't source the install script in webpage.sh

[yubiuser]

What does this PR aim to accomplish?:

While working on the installation script locally, I noticed a sourcing loop which prevents testing certain changes to the install script.
During installation, we source webpage.sh

pi-hole/automated install/basic-install.sh

Line 1857 in d885e92

source "${PI_HOLE_LOCAL_REPO}/advanced/Scripts/webpage.sh"

And one of the first things we do in webpage.sh is to source the install script from the repo
https://github.com/pi-hole/pi-hole/blob/d885e92674e8d8d9a673b35ae706b2c49ea05840/advanced/Scripts/webpage.sh#L27C8-L29

This leads to overwriting any function in the install script that initially called webpage.sh by the functions defined in the repo (which is stock master branch). Hence, downstream modifications to the install script will be reset.

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We source the installer script inwebpage.sh for a few variables and valid_ip() and valid_ip6().I tried to move the functions to utils.sh but this does not work because the installer requires the function early and before we download the utils.sh script.

How does this PR accomplish the above?:

Duplicate valid_ip() and valid_ip6() into webpage.sh and remove sourcing of the installer script.

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[PromoFaux]

Can I suggest that if we're going to be duplicating them, we duplicate them into utils.sh anyway, and source that in webpage.sh? webpage.sh will be disappearing in v6 I think (It's been a while since I worked on the development-v6 branch in anger, so I may not be as up to speed as I remember!)

[yubiuser]

webpage.sh will be disappearing in v6 I think

This is the reason I would not put in utils.sh- it will be gone in v6 "automatically". If it would be in utils.sh we would need to remember to remove it.

[DL6ER]

This file will be largely removed in v6.0, I'm not sure what we will keep else than a convenience CLI wrapper for setting the API password.