Don't make sheet name an input column, go back to special casing it on row create

This means it won't update if put in the wrong place,
but avoids issues with reverse sync trying to write it out when it's not an actual column

sheetsync/sheetsync/main.py

@@ -77,7 +77,6 @@ class SheetSync(object):
 		self.create_missing_ids = False
 		# List of input columns
 		self.input_columns = [
-			'sheet_name',
 			'event_start',
 			'event_end',
 			'category',
@@ -182,7 +181,7 @@ class SheetSync(object):
 			logging.info("Inserting new event {}".format(row['id']))
 			# Insertion conflict just means that another sheet sync beat us to the insert.
 			# We can ignore it.
-			insert_cols = ['id'] + self.input_columns
+			insert_cols = ['id', 'sheet_name'] + self.input_columns
 			built_query = sql.SQL("""
 				INSERT INTO events ({})
 				VALUES ({})