fix(plugins):Call NotifyPlugins task regardless of metadata sync success

- Before this commit, if any SyncbackMetadataTask for a message failed, we would
never run NotifyPluginsOfSendTask because of the dependency in place.
This caused unintended consequences like open/link tracking not working
if any other plugin failed to syncback metadata, despite the tracking
metadata having been saved successfuly.

- This commit makes it so NotifyPluginsOfSendTask always runs after
the SyncbackMetadata tasks regardless of wether they fail or succeed by
updating the task queue to support this dependency scenario

src/flux/stores/task-queue.coffee

@@ -247,7 +247,7 @@ class TaskQueue
 
   # Recursively notifies tasks of dependent errors
   _notifyOfDependentError: (failedTask, err) ->
-    downstream = @_tasksDependingOn(failedTask) ? []
+    downstream = @_tasksToDequeueOnFailure(failedTask) ? []
     Promise.map downstream, (downstreamTask) =>
 
       return Promise.resolve(null) unless downstreamTask
@@ -280,9 +280,11 @@ class TaskQueue
       otherTask.queueState.debugStatus = Task.DebugStatus.DequeuedObsolete
       @dequeue(otherTask)
 
-  _tasksDependingOn: (task) ->
+  _tasksToDequeueOnFailure: (failedTask) ->
     _.filter @_queue, (otherTask) ->
-      task isnt otherTask and otherTask.isDependentOnTask(task)
+      failedTask isnt otherTask and
+      otherTask.isDependentOnTask(failedTask) and
+      otherTask.shouldBeDequeuedOnDependencyFailure()
 
   _taskIsBlocked: (task) =>
     _.any @_queue, (otherTask) ->

src/flux/tasks/notify-plugins-of-send-task.es6

@@ -62,6 +62,10 @@ Any plugins you used in your sent message will not be available.`
     return (other instanceof SyncbackMetadataTask) && (other.clientId === this.messageClientId)
   }
 
+  shouldBeDequeuedOnDependencyFailure() {
+    return false
+  }
+
   performLocal() {
     this.validateRequiredFields([
       "messageId",

src/flux/tasks/task.es6

@@ -513,6 +513,14 @@ export default class Task {
     return false;
   }
 
+  // Public: determines if the current task should be dequeued if one of the
+  // tasks it depends on fails.
+  //
+  // Returns `true` (should dequeue) or `false` (should not dequeue)
+  shouldBeDequeuedOnDependencyFailure() {
+    return true;
+  }
+
   onDependentTaskError(other, error) {
 
   }