adds selection for web root and admin interface subdirectory

[Gezzo42]

• temporary changes GitHub repository

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What does this PR aim to accomplish?:

Pihole should be able to install the web interface at a different location than /var/www/html/admin, like /var/www/html/pihole.

How does this PR accomplish the above?:

I changed the hard coded paths for /var/www/html and admin in the pihole scripts with the two additional variables WEB_ROOT and ADMIN_INTERFACE_SUBDIR. They are set to the current values as default and a added dialog allows to change them. Also a single adaption to the pihole web repository was necessary, which later needs also a pull request.
At the moment the adaptions to the tests are missing and I also changed the Github links to test this.

Can you please give me some feedback, if my current implementation is OK or what needs to be changed!? Also help/hints for the tests would be nice.

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

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