devise: fixing the remember_me option.

Setting remember_me in the controller did not work.
Therefore, we are setting this option in the model, now.

This results in the desired behaviour to keep the user logged in after restarting the browser.
devise fix: after clicking a link without being signed in, the user should be asked for his login and then being *redirected to the link url*.

Before this fix, the user has always been redirected to the root_path. Now, he's redirected properly.

Wiki: https://github.com/plataformatec/devise/wiki/How-To:-Redirect-back-to-current-page-after-sign-in,-sign-out,-sign-up,-update