notebook: relocate to %load_ext tensorboard

[wchargin]

Summary:
“Drop the .notebook. It’s cleaner.” (Fixes #1857.)

Using %load_ext tensorboard.notebook now gives users a helpful error.

This is a breaking change, which we think is okay because (a) notebook
support was always explicitly marked as “alpha” in release notes and
“experimental” in the code, and (b) notebooks are meant to be run
interactively, so the fix will be easy and immediate.

Test Plan:
Built the Pip package and installed it into a new virtualenv, then
verified that %load_ext tensorboard.notebook properly emits an error
message, which directs users to the working %load_ext tensorboard:

Tested the above with tf-nightly and tf-nightly-2.0-preview. Also
verified that the getting_started.ipynb notebook still works, from
start to finish, within the new virtualenv.

Verified that there are no lingering references to the old endpoint:

$ git grep '%load_ext tensorboard.notebook'
tensorboard/notebook.py:      "Use '%load_ext tensorboard' instead of '%load_ext tensorboard.notebook'."

wchargin-branch: notebook-extension-at-root

[review-notebook-app]

Check out this pull request on ReviewNB: https://app.reviewnb.com/tensorflow/tensorboard/pull/2119

Visit www.reviewnb.com to know how we simplify your Jupyter Notebook workflows.

[nfelt]

A million dollars isn't cool. You know what's cool?

%load_ext tensorboard