Milestone 2 - Added accounts

[Duta-Sebastian]

Title
Implement Database Connections, User Authentication, and Logging

Description

Account interface and 3 types of accounts.
The app can now store 3 types of accounts for each user ( bank accounts, email accounts and social media accounts ), each with their own attributes. The account handling logic is always implemented using a pointer to the interface that all the types inherit from.

Account Factory
An account factory static class has been implemented to make it easier to instantiate each type of account.

Testing:
Added unit tests for all account handling logic, for the account factory and for the overloaded + operator on the std::vector template.

Rationale
This is the main part of the application. This is where all the account logic is handled. Errors were refactored.