Add Sign In view

[Eghinner]

Description

To adapt to the changes requested by the Apple App Store review team, it is necessary to add a functional view that shows that the registration is done without purchase.

It is also intended to attract users interested in becoming partners.

Tech details

The change consists of adding changes that adapt to the previously created endpoints to register users/members in the admin panel.

NOTE: you may need to adapt the a little bit the controller to this form, but it is not necessary to create a new one.