Use `scroll_to` helper instead of relying on a large viewport

Description

We are currently relying on a very large viewport, to make it possible to click buttons / things that are at the bottom of a page.

But this is not enough, because we have pages that are very long (like Admin -> Settings).

Proposal

Use scroll_to helper:

def save_settings
  scroll_to '.form-actions' do
    click_button 'Save'
  end
end

# Implementation in Page::Base

def scroll_to(css, &block)
  page.execute_script <<~JS
    document.getElementsByClassName("#{css.sub(/^\./, '')}")[0].scrollIntoView();
  JS

  page.within(css, &block)
end

It works, but we can improve this helper even further.

This is implemented in Geo tests MR (not pushed yet).

/cc @rymai @stanhu