Recover embed examples

fixes #1458 

This recover the embed examples: 2 scripts and the reveal-based flask-based embed-based application :wink:

I just opened this as an standalone PR because it is a group of related examples and I am reintroducing them as a whole, not just tiny fixes like we will push to the pre-release example PR.
> Just as a suggestion, maybe setting a global pop = None with a comment that it'll be properly set later (or turning it a local variable) makes easier to understand?

It is a good suggestion... there is a tension on this example... because I want to show that you can take isolated blocks of code (featuring different kins of Bokeh visualizations) and rearrange them in a bigger flask-based app without losing the independence of each example (this is why there are import in so weird places and other things that usually I would write them in another way). I think this "building blocks" approach has some conceptual advantages for people trying to quickly understand, and more importantly, use the embed API. Just some thoughts to give you some context :wink: