Orders & Inventory tables

[gassc]

Implement two new tables:

• an Order table: for recording new stock that's been ordered/purchased for sale. This workflow for this table, as far as Flask-Admin is concerned will be similar to that of the Sale table.
• an Inventory table: tabulate what products are in stock by populating the content of this table via triggers fired by inserts/updates to the Sale and Order tables.

Question(s) for test users:

• how easiest to record purchased stock? by unit price (so, total cost is that * quantity), or invoice price (so, decoupling list price from purchase price; may be case for bulk orders).