Move System::jsonExit to BaseModule->jsonExit

- This will ensure headers set in BaseModule->run will be carried in jsonExit scenarios
- Deprecate jsonExit() method in Core\System

src/Module/Api/Mastodon/Accounts.php

@@ -58,6 +58,6 @@ class Accounts extends BaseApi
 		}
 
 		$account = DI::mstdnAccount()->createFromContactId($id, $uid);
-		System::jsonExit($account);
+		$this->jsonExit($account);
 	}
 }