Make the pages' titles configurable

The attached patch add the functionality.
Incidentally the page--user--password.html.twig and page--user--register.html.twig wouldn't be anymore necessary right now, but I had not removed them as they could be useful for unforeseen future changes.

We should keep the module as tidy as possible so I'll close this.
As an alternative idea you should be able to change the titles by changing them in the interface translation.

I get your point, however that is useful only for translated page, not for the main language, am I wrong'