Improve history event view for activity with retry that is started

[longquanzheng]

This will reduce the confusion like uber/cadence#3802

We already have the

Option 1: Append dummy activity started event in history for activity with retry that is started
Option 2: Do something like CLI to show the started activities at the end of the history.

============Pending activities============
[
  {
    "ActivityID": "0",
    "ActivityType": {
      "name": "main.batchProcessingActivity"
    },
    "State": "STARTED",
    "LastStartedTimestamp": "2020-10-11T22:47:16-07:00",
    "LastHeartbeatTimestamp": "2020-10-11T22:47:16-07:00",
    "Attempt": 0,
    "MaximumAttempts": 15,
    "ExpirationTimestamp": "2020-10-11T22:47:36-07:00"
  }
]
NOT

[just-at-uber]

We are proposing a new feature to create a pending tab with a pending activities button at the bottom of history which will take users to the pending tab that will show all pending activites, decisions, children, and potentially timers in the future.