emoji_eventsFlatten nested settings

Config files are often nested. Write flatten(d) that turns nested dicts into one level, joining keys with dots:

flatten({"db": {"host": "x", "port": 5432}, "debug": True})
# {"db.host": "x", "db.port": 5432, "debug": True}

• Only dicts are flattened; lists and other values are kept as they are.
• An empty nested dict disappears.