Create Polymer UI for copypaste sample

Assigning HTML elements to global variables seems like a bad idea to me, some reasons: 
- it means that copypaste-getter is not actually a re-useable component (multiple components will overwrite the same globals)
- it makes code outside this component responsible for updating the HTML display, which would be better encapsulated in this file.

I think a better way to do this is to get rid of these global variables, and use bindings to update the properties you need.  For example with the step2ContainerNode element, you change the style.display property to block (it's initialized as hidden) in popup.js.  Instead it would be better to have some model maintained by popup (could be an object called model like in uProxy, or a bunch of globals, or something passed in as an attribute to coyppaste-getter) - you could then bind HTML in this template to that model.  e.g. in popup.js:
```
var model = { readyForStep2: false };
...
freedom.on('signalForPeer', (signal:WebRtc.SignallingMessage) => {
  model.readyForStep2 = true;
...
```
then in getter.html change this div from:
```<div id='step2ContainerNode' style='display: none;'>```
to:
```<div id='step2ContainerNode' style='display: {{ model.readyForStep2 ? "block" : "none" }};'>```