Remove on_stock from update form.

The value on_stock should only be set via a new transaction so it's always clear what happend with the parts
Added a comment field to Transaction-model.

This is for saving maybe some more information that are not fitting into subject line
Added a view to add a new transaction.

Currently no effect on on_stock. Also during this, user field has ben renamed to created_by to be consistent via application and field unit has been removed as not needed
Add link to new transaction page to startpage if user is logged in
Use generic ListView class for listing all parts instead of modifying plain view class
Adding a View to show all transactions taken place
Added link to transaction list to main page