Laravel components v8 (#2576)

- update actions ci
- include json for 4 spaces tab
- provide output int for process code exit
- adhere to parent type hint of builder
- mailer instance now needs a name, multiple can be instantiated
- getOriginal now uses mutators in the model
- Temporarily loosen MailableInterface requirements. This avoids an immediate BC break for classes in extensions that implement this interface.
- Temporarily provide (and autoload) old symfony translator interface
- make queue exception handler compatible with the contract of L8
- Update phpunit schema for newer version
- Update phpunit assert calls for newer version

tests/phpunit.integration.xml

@@ -1,22 +1,24 @@
 <?xml version="1.0" encoding="UTF-8"?>
-
-<phpunit backupGlobals="false"
-         backupStaticAttributes="false"
-         colors="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
-         processIsolation="true"
-         stopOnFailure="false">
-
+<phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
+    backupGlobals="false"
+    backupStaticAttributes="false"
+    colors="true"
+    convertErrorsToExceptions="true"
+    convertNoticesToExceptions="true"
+    convertWarningsToExceptions="true"
+    processIsolation="true"
+    stopOnFailure="false"
+>
+    <coverage processUncoveredFiles="true">
+        <include>
+            <directory suffix=".php">../src/</directory>
+        </include>
+    </coverage>
     <testsuites>
         <testsuite name="Flarum Integration Tests">
             <directory suffix="Test.php">./integration</directory>
         </testsuite>
     </testsuites>
-    <filter>
-        <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">./src/</directory>
-        </whitelist>
-    </filter>
 </phpunit>