Group create dialog: only enter localpart

Since we currently can only create groups on the local server anyway,
there's no point making the user jump through the hoop of forming the
whole group ID and telling them off if they got the server name wrong.

src/i18n/strings/en_EN.json

@@ -590,7 +590,7 @@
     "Community Name": "Community Name",
     "Example": "Example",
     "Community ID": "Community ID",
-    "+example:%(domain)s": "+example:%(domain)s",
+    "example": "example",
     "Create": "Create",
     "Create Room": "Create Room",
     "Room name (optional)": "Room name (optional)",