Only display active dashboards

[wchargin]

Summary:
This commit incorporates the backend's is_active plugin functions and
the related plugins_listing handler, and uses them on the client: we
will only display dashboards that correspond to backend plugins that are
active.

Test Plan:

• Launch TensorBoard with a normal data set and note that the correct
  set of dashboards is displayed.
• Launch TensorBoard with a nonexistent or empty logdir and observe
  the global "no data" message.
• Launch TensorBoard and cause the plugins_listing network request
  to fail. (In Chrome: Network devtools tab, right-click the request,
  select "Block Request URL", refresh.) Note the resulting message.
  Then, unblock the request, and click TensorBoard's reload button:
  everything should work as usual.
• Launch TensorBoard with a large data set, and open the frontend
  before it's read any text summaries. Then, click TensorBoard's
  reload button and note that the "Text" dashboard tab appears and
  functions.
• Launch TensorBoard on a large data set. Select the "text" dashboard.
  Then, relaunch the TensorBoard backend on the same port, and click
  the frontend's reload button before any text summaries are loaded.
  Note that TensorBoard removes the text dashboard and shoves the user
  over to a different dashboard (the first dashboard loaded) with a
  notice.
• Launch TensorBoard, let it load data, then kill the server and click
  TensorBoard's reload button. Note that the already-loaded features
  of TensorBoard are still operational.

wchargin-branch: active-dashboards

[teamdandelion]

Very nice! Some changes requested:

• Keep a list of inactive dashboards, as a dropdown on the right ('5 inactive dashboards') and let user explicitly load it if they want.
• If user puts hash for inactive dashboard, nav to it anyway.
• Make font consistent for the "loading dashboards" message in top bar.
• If we keep the toast, it should be on top not bottom of window.
• Fix the broken logdir test.

[wchargin]

Thanks for the review! I'll post a comment here when I've addressed each of these.

• Keep a list of inactive dashboards, as a dropdown on the right ('5 inactive dashboards') and let user explicitly load it if they want.
• If user puts hash for inactive dashboard, nav to it anyway.
• Make font consistent for the "loading dashboards" message in top bar.
• If we keep the toast, it should be on top not bottom of window.
• Fix the broken logdir test.

[wchargin]

Okay! Changes summarized below.

I've removed the toast; you can now navigate to a nonexistent plugin and you'll receive a standard warning:

Inactive dashboards are presented in a separate dropdown and may be selected:

If you're focused on an inactive dashboard, then you use TensorBoard's reload button, and the dashboard becomes active, then you'll still be focused on it as it moves to the list of active dashboards.

The logdir is shown when there is a warning message, which fixes the test.

[wchargin]

Per your request, I've noinked the dropdown. (Personally, I prefer it with ink because of the immediate feedback; the UI feels higher-latency to me without the ink. But I'm fine with using noink.)

The focus state on the dropdown items is required for a11y.

PTAL :-)

[teamdandelion]

Discussed offline:

• Some reformatting of the "inactive dashboards" dropdown. It would be nice if it were totally cohesive with the tabs.
• Let's add some WebDriver testing :)

[wchargin]

Enjoy! I've included a WebDriver test that I think is reasonably comprehensive. (Let's see what Travis thinks…)

[wchargin]

(Last push was just a rebase; no content change.)