Need a way to find enclosing cell from a dom element on the page

For jupyter-incubator/declarativewidgets/issues/125 we needed a way to get a hold of the enclosing cell of one of our elements to then have the correct callbacks to handle messages like `display_data`.

We looked around and could not find a better way than to do `$(this).parents(".cell").data().cell` (obviously with the right checks).

It seems that we should have a utility method that would do this for us somewhere in IPython.notebook. Also, it should be something that would allow Thebe-like environments to also work (something as simple as a well-known method that could be redefined by the environment).

/cc @Carreau

Start by inspecting the IPython.notebook APIs and the existing $(this).parents(".cell").data().cell lookup described in the issue. Define the utility's behavior for finding an enclosing cell and its override point for Thebe-like environments; done means both notebook and redefined-environment lookups are supported.

Written by the indexing model from the issue text.