Add an API endpoint: "GET /accounts/"

[simeonkrastev]

• [x] Create a new controller, called AccountsController
• [x] Make an HttpGet method that returns a list of all accounts in the database
• [x] Make an HttpGet /accounts/{id} endpoint that returns the account data for a given account
• [x] Make an HttpPost endpoint that adds a new account.
• [x] Make an HttpDelete endpoint that removes an account by ID.