✨ AddUserModal + UserDetail - optional username field

ref #12

src/locales/en.json

@@ -117,6 +117,7 @@
     "track-length-in-m": "Track Length in m",
     "track-name": "Track name",
     "tracks": "Tracks",
+    "username": "Username",
     "users": "Users",
     "valid-email-is-required": "valid email is required",
     "welcome_wavinghand": "Welcome 👋",