visibility: make `tensorboard.context` public (#4886)
visibility: make `tensorboard.context` public (#4886)

Summary:
The `RequestContext` type is already public via `plugin_util.context`.
Some plugins need to access it directly to create a background context,
as done in the projector plugin or tensorflow/profiler#298. This module
is designed as a public API, so it can be made globally visible.

Googlers, see <http://cl/368909135>.

wchargin-branch: context-public

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard