Seamless entity reference autocomplete integration

I think we should try to get the updated for 8.x-2.x.

This patch adds one more feature - "Advanced search" link at the end of autocomplete suggestions. When clicked, it opens entity browser.

I added preview improvement. So now, when something is selected you can setup preview of it in widget configuration. How it looks like:

Re-rolled against latest 2.x + fixed an issue when copy/paste something in the autocomplete field (this issue was introduced in patch in #31).