Update ImageProxy to pass PHPStan Level 9

[Alkarex]

#185 (comment) and follow-up of #201

Only a couple of files are lacking a little update to pass PHPStan Level 9 for all this Extensions repository:

./xExtension-ImageProxy/configure.phtml
./xExtension-ImageProxy/extension.php

The problem seems to be that ImageProxy saves the configuration parameters in FreshRSS top level instead of one level down dedicated to extensions

How to work:

git clone https://github.com/FreshRSS/FreshRSS.git
git clone https://github.com/FreshRSS/Extensions.git
cd Extensions/
make test-all
./vendor/phpstan/phpstan/phpstan analyse --level 9