Fix app config

- Add missing content in __init__.py
- Rename app config to mensa and add verbose_name

aleksis/apps/mensa/__init__.py

+import pkg_resources
+
+try:
+    __version__ = pkg_resources.get_distribution("AlekSIS-App-Mensa").version
+except Exception:
+    __version__ = "unknown"
+
+default_app_config = "aleksis.apps.mensa.apps.MensaConfig"

aleksis/apps/mensa/apps.py

-from django.apps import AppConfig
+from aleksis.core.util.apps import AppConfig
 
 
-class MenuConfig(AppConfig):
-    name = 'menu'
+class MensaConfig(AppConfig):
+    name = "aleksis.apps.mensa"
+    verbose_name = "AlekSIS – Mēnsa (cantina menu)"