(Fix): method getUserByUsername
retrieves the user even if there are no associated accounts.

(Add method): getAccountsList of User class returns a list of accounts currencies.
Add(.gitignore): exclude .idea/ directory from tracking.
Refactor(DBConfiguration): moved to package configuration.
Refactor(SignInServlet): moved to package authentication.
Add(UserAuthenticator): method signIn
comparing username and password to those from database.