Provide a foolproof way to unmasquerade

Temporary patch updated with "Switch Back" link changed to use new `/unmask` route in `masquerade_switch_user_validate()`.

Patch from #17 works.
I think this is a good approach as other workarounds are not as simple as this one.

I don't know if this problem was solved, but I simply did this:
I placed the "masquerade" block where I wanted the switch-back link, hid the user search form via CSS (for switching, I prefer the Masquerade Field approach and views), and styled only the switch-back link, without modifying the form.