Create Data Repository Layer for More Precise Testing + Documentation

[pwdel]

Re, data conventions in readme:

Data Conventions

• The entire application should be as stateless as possible, meaning we have a one-way writeable database of users, markets, bets and calculate all relevant states of the application from as few possible columns within those models.
• We should separate the data logic from the business logic as much as possible with a Domain Driven Design (DDD), meaning we have a repository/ directory which is designed to be the central location to keep functions that extract data from the databases. This should ideally help slow the growth of the codebase over time and keep data extraction more testable, which should make our startless architecture more reliable.