Only display active dashboards

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.

Also, `bazel run //tensorboard/functionaltests:core_test` and step away
from your computer for a minute or so.

wchargin-branch: active-dashboards

tensorboard/components/tf_backend/router.ts

@@ -20,6 +20,7 @@ export type RunTagUrlFn = (tag: string, run: string) => string;
 export interface Router {
   logdir: () => string;
   runs: () => string;
+  pluginsListing: () => string;
   isDemoMode: () => boolean;
   pluginRoute: (pluginName: string, route: string) => string;
   pluginRunTagRoute: (pluginName: string, route: string) => RunTagUrlFn;
@@ -60,6 +61,7 @@ export function createRouter(dataDir = 'data', demoMode = false): Router {
   return {
     logdir: () => dataDir + '/logdir',
     runs: () => dataDir + '/runs' + (demoMode ? '.json' : ''),
+    pluginsListing: () => dataDir + '/plugins_listing',
     isDemoMode: () => demoMode,
     pluginRoute,
     pluginRunTagRoute,

tensorboard/components/tf_storage/storage.ts

@@ -311,8 +311,8 @@ function _componentToDict(component: string): StringDict {
   const tokens = component.split('&');
   tokens.forEach((token) => {
     const kv = token.split('=');
-    // Special backwards compatibility for URI components like #events
-    if (kv.length === 1 && _.contains(TABS, kv[0])) {
+    // Special backwards compatibility for URI components like #scalars.
+    if (kv.length === 1) {
       items[TAB] = kv[0];
     } else if (kv.length === 2) {
       items[decodeURIComponent(kv[0])] = decodeURIComponent(kv[1]);