Add clean UI for plugins requiring TensorFlow when not installed

[orionr]

As a part of supporting non-TensorFlow installs of TensorBoard, we need to disable UI for plugins that require TensorFlow and can't be loaded. One option is to take inspiration from the debugger plugin and show a popup with the following:

• Title: This plugin is disabled without TensorFlow
• Message: To enable this plugin in TensorBoard, install TensorFlow with pip install tf-nightly

Once we've done that we can also remove the requirement in #1796 that non-TF needs to be run with

bazel build tensorboard
TENSORBOARD_NO_TF=1 ./bazel-bin/tensorboard/tensorboard --logdir ~/local/sample_runs/runs

and instead will simply be runnable with

bazel build tensorboard
./bazel-bin/tensorboard/tensorboard --logdir ~/local/sample_runs/runs

[orionr]

cc @lanpa, @nfelt