Modify the optin to not leave the screen unless all the mandatory fields are filled up

[miguelgfierro]

Currently, if there is a mandatory field that is not filled up, sendy sends you to a different page, and then the cookie is already created, so when you go back, there is no optin.

Two options:

1. Create a JS that modifies the optin, sending an error message
2. Modify the cookie strategy to only be saved when a person submits a correct form or a person presses the cross