layout db: Entries created using the LTF interface are not always editable with the python interface

In the Layout DB, it is not always possible to edit certain fields using the python interface (`peewee`) if they were created using the LTF interface.

For example, see the entry with component name `allenby_cylinder` created by @mondana using the LTF text interface. It is not possible to add/sever connexions for this entry (and others created using LTF). This is just one example, but this issue is general.

If the components are added using python (`peewee`), then there is no issue.

```
Traceback (most recent call last):
File "add_allenby_outrigger_components.py", line 45, in
if __name__ == main():
File "add_allenby_outrigger_components.py", line 42, in main
fix_allenby_cylinder_name() # Fix barcode name of allenby cylinder
File "add_allenby_outrigger_components.py", line 36, in fix_allenby_cylinder_name
layoutdb_utils.change_component_serial_num(serial_num, new_serial_num, user_name)
File "../utilities/layoutdb_utils.py", line 86, in change_component_serial_num
component_id.save()
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 6532, in save
rows = self.update(**field_dict).where(self._pk_expr()).execute()
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 1898, in inner
return method(self, database, *args, **kwargs)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 1969, in execute
return self._execute(database)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 2465, in _execute
cursor = database.execute(self)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 3142, in execute
return self.execute_sql(sql, params, commit=commit)
File "/data/user-data/pearlman/.conda/envs/layoutdb/lib/python3.7/site-packages/chimedb/core/connectdb.py", line 253, in execute_sql
cursor = super(RetryOperationalError, self).execute_sql(sql, params, commit)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 3136, in execute_sql
self.commit()
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 2902, in __exit__
reraise(new_type, new_type(exc_value, *exc_args), traceback)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 185, in reraise
raise value.with_traceback(tb)
File "/home/pearlman/.local/lib/python3.7/site-packages/peewee.py", line 3129, in execute_sql
cursor.execute(sql, params or ())
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/cursors.py", line 148, in execute
result = self._query(query)
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/cursors.py", line 310, in _query
conn.query(q)
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/connections.py", line 548, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/connections.py", line 775, in _read_query_result
result.read()
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/connections.py", line 1156, in read
first_packet = self.connection._read_packet()
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/connections.py", line 725, in _read_packet
packet.raise_for_error()
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/protocol.py", line 221, in raise_for_error
err.raise_mysql_exception(self._data)
File "/home/pearlman/.local/lib/python3.7/site-packages/pymysql/err.py", line 143, in raise_mysql_exception
raise errorclass(errno, errval)
peewee.IntegrityError: (1451, 'Cannot delete or update a parent row: a foreign key constraint fails (`ch_data`.`connexion`, CONSTRAINT `connexion_ibfk_2` FOREIGN KEY (`comp_sn1`) REFERENCES `component` (`sn`))')
```