More features for stockline notes

[sde1000]

The stockline notes feature has been very useful at EMF.

We have a config setting to say whether the note for a stockline should be cleared when a new item is put on stock. (At EMF this is set to "yes".)

For some stocklines it would be useful for putting a new item on sale to set a note by default, eg. "Needs pulling through". Maybe add a column to Stocklines for default notes on new stock? We could then remove the config setting. (There would be a difference between a null new note and a blank new note — null would mean "don't touch the note" and blank would mean "clear it".)