[Backend] As a user, I can register a new account with email and password

[vnntsu]

Why

• Only authenticated users can interact with our application. Beforehand, they must register a new account.
• User can register by email and password.

Acceptance Criteria

• Store registered users to the database:
  • Create a new database table for storing user credentials.
  • Add a context.
  • Add a schema and changeset.
• Password will be saved in an encrypted state
• Email is a unique field, users can't register multiple accounts with the same email