Create a basic schema for storing trading data

[DanielPointon]

We'll need to store data on stock prices and purchases over time in the database. This will require a database schema. I'm guessing:

• stocks table (ID, stock_name, exchange)
• stock_values (stock_id, value, time)
• user_purchases (id, user, stock_id, quantity)
• users (id, email, password_hash) - can be extended if we want profiles etc
• user_portfolio_entries (id, user_id, stock_id, quantity)

Note duplication of data between user_purchases and user_portfolio_entries is to avoid recalculating a user's current portfolio based on their historical purchases every time they load the app. Can be thought of as a changelog.