[Dynamic plugin] tracking bug

We are building the dynamic loadable plugin system as per described in https://github.com/tensorflow/community/pull/90. This is a task tracking bug for those who are interested in following the status.

• Infrastructural changes
  • plugin detection and loading: #2257
  • frontend loading of the plugin resources: #2318
  • misc: #2304, #2334, #2329, #2317, #2340,
• create a "Hello World" of the dynamic plugin: #2354, #2374
• write a test for the demo dynamic plugin API: #2354
• document the changed plugin system: #2354, #2389
• convert one of the TensorBoard plugin as a dynamic plugin: #2356
• plugin developer UX guidelines
• create a plugin library that has access to tf_storage and foundational components like runs selector
  • tf_storage iframe implementation
  • library bundling