Rework database to accommodate multiple securities

[donjar]

The database schema is built on the assumption that there is only one security (i.e. Grab shares). If we are going to have more than one securities traded in the platform, we need to rework it. Some things in mind:

• Join users with securities table, and put can_buy, can_sell, is_committee information in the join table. Otherwise, a user can either buy all securities or not buy anything.
• Update the matching engine to accommodate this.
• Modify the chat tables, possibly.