Add tensorboard-notf target for non-TensorFlow build

[orionr]

A continuation of #1401 to see if we can get CI to compile. Many thanks to @ml7 for making this possible!

This will follow #1451 and #1453

To test (I ran this on a Mac, but Linux is likely similar) run the following

conda create --name notf python=2.7  # Note that we are using a conda environment without TensorFlow to test
conda activate notf
pip install --upgrade pip
conda install protobuf
conda install absl-py
conda install numpy
bazel build tensorboard:tensorboard-notf --verbose_failures
# Sample runs directory comes from `git clone https://github.com/orionr/sample_runs`
./bazel-bin/tensorboard/tensorboard-notf --logdir ~/local/sample_runs

cc @jspisak @jart @martinwicke @nfelt

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of all the commit author(s), set the cla label to yes (if enabled on your project), and then merge this pull request when appropriate.

[orionr]

@ml7 looks like I might need some approval from you here. :) Still, let's see how CI does.

[ml7]

ok with PR

[googlebot]

CLAs look good, thanks!

[orionr]

@nfelt this should be ready for review whenever you are. Thanks.

[orionr]

Thanks for the quality review. Updates made.