Improve the gender selection on the register page

[wintermeyer]

Move the gender selection up in the form (red):

And now add a little bit of LiveView magic. The aim is to display the best "Could interest you" selection possible (blue). We start with a mix of male and female. But when somebody changes the gender value we can assume that males only search for females and vice verse (for now we don't change the content for divers). So when that field is changed we should change the content of "Could interest you" accordingly.

And if somebody doesn't change the gender but changes the birthdate, than we can assume that the current gender is the right one and update the "Could interest you" accordingly (e.g. displaying only females).

Bonus: It would be nice if we could make this a bit cooler. I am thinking about removing the not matching entries with a little animation and adding the matching ones in the bottom of the list. So that anybody sees that there was a change and that the new selection makes more sense for him/her.