docs(examples): use form POST to create Plunkers

The previous solution for opening Plunkers from the docs relied on tight
coupling between the docs site and the plunkr site, in particular the
URL to the example code on the docs server was hard coded in the Plunker
site.

This change goes back to the old POST method of creating a Plunker, but
with a subtle difference: In the very old docs, the content was injected
directly into the example HTML at build time.  This was easy enough to
do as the example actually ran in the current page but also increased
the size of the doc page.

The new examples are run in completely separate iframes. This new version
of showing a Plunker loads the file content for the Plunker from the
server by accessing the example's manifest.json file using $http requests.

This also has the additional benefit that you can now generate plunkers
from examples that are running locally or, frankly, in any folder on any
server, such as personal builds on the Jenkins CI server.

Closes #7186