Add a POST /password/reset endpoint

[begedin]

We need to add a POST /password/reset endpoint as the last step to reset a user's password

• Takes in [:value, :password, :password_confirmation]
• Finds AuthToken by :value, as well as associated User
• Uses [Phoenix.Token.verify/4] to verify the token is correct and not expired or anything like that.
• If all is in order, updates the user with new password
  • For this, w need a User.reset_password_changeset which casts and validates :password and :password_confirmation. It should ensure the two match before encrypting and assigning the new password. This can be done as a separate issue in Create User.reset_password_changeset/2 #728
  • Endpoint should render validation errors if the changeset is invalid

Subtasks

• add endpoint to router
• add PasswordResetController.reset_password
• past this point, we are blocked by Create User.reset_password_changeset/2 #728
• ensure endpoint renders 200 OK if reset passes
• ensure endpoint renders 422 if reset fails due to password mismatch
• add controller tests

References

Progress on: #498