Move to Symfony Flex

The structure changed completely.
Bundles are gone. Everything is flatten in the folder `src`.
I separated import & api controllers to avoid _pollution_ in the main controller folder.

config/packages/dev/framework.yaml

@@ -0,0 +1,9 @@
+framework:
+    router:
+        strict_requirements: true
+    profiler:
+        only_exceptions: false
+
+    mailer:
+        # see https://mailcatcher.me/
+        dsn: smtp://127.0.0.1:1025