Move custom tooltip on payment page to a modal on mobile

It also fix the size of the total input. 

Closes #336.
I started creating a helper for this logic, but since I didn't needed in the part I was implementing, I unstaged.

```ruby
def small_screen?
  browser.mobile? || browser.tablet?
end
```

This is important because the [main definition of "mobile"](http://dictionary.reference.com/browse/mobile) does not came with the idea of small screens which we use in this HTML.