Add compatibility with PHP 8 (see #2535)

Description
-----------

See #2263 and #2291

### TODO

* [x] Fix the tests.
* [x] Wait if sebastianbergmann/php-code-coverage#837 gets merged and then use PHPUnit 8.5 for all PHP versions.
* [ ] <del>Click through the back end and front end and fix all warnings.</del>

Commits
-------

8ad61492 Add PHP 8.0 to the CI chain
7b8729e5 Fix the unit and functional tests
47c14dbb Use 127.0.0.1 instead of localhost in the functional tests
938836c0 Always set the ACCEPT_LANGUAGE header in the functional tests
a4199ec2 Use PHPUnit 8.5 for all PHP versions
7ffbd267 Adjust the error level in PHP 8
4ce6b27b Merge branch '4.9' into fix/php8

Author: leofeyer

composer.json

@@ -15,7 +15,7 @@
         }
     ],
     "require": {
-        "php": "^7.2",
+        "php": "^7.2 || ^8.0",
         "contao/core-bundle": "self.version",
         "patchwork/utf8": "^1.2",
         "symfony/config": "4.4.*",
@@ -30,7 +30,7 @@
     "require-dev": {
         "contao/manager-plugin": "^2.3.1",
         "contao/test-case": "^4.0",
-        "phpunit/phpunit": "^8.4",
+        "phpunit/phpunit": "^8.5",
         "symfony/http-client": "4.4.*",
         "symfony/phpunit-bridge": "4.4.*"
     },