Add instructions for using the Due Date / Reminders apps

The following discussions from !81983 (merged) should be addressed:

• @mdunninger started a discussion:

  1. Occasionally you will send a customer a reply letting them know that you're going
     to follow up with further information.
     1. In this case you should set the ticket status to `Open` or `On-hold`. Either
        way ZenDesk still removes the SLA and assigns the ticket to you (if it's not
        already assigned to someone else).
     1. In your reply you should also inform the customer when to expect your follow-up
        message, and invite them to inform you if your chosen schedule does not meet
        their needs. If it does not, please take steps to ensure you understand the
        impact to their business so that you can adjust your plans accordingly.
     1. To help yourself with meeting any schedule expectation you've set, consider
        using our [Due Date](https://about.gitlab.com/handbook/support/support-ops/documentation/zendesk_global_apps.html#due-date-picker)
        and [Reminders](https://about.gitlab.com/handbook/support/support-ops/documentation/zendesk_global_apps.html#gitlab-reminders-app)
        apps.
  1. For any ticket in which the user is waiting on a response from Support, **always
     provide a timely update** on its status and on the work that has been done. You
     are encouraged to set your own definition of "timely" by setting the customer's
     expectations and using the Due Date and Reminders apps, as described in the
     preceding tip. Otherwise, please aim to provide updates daily, and definitely
     no less than every four days, which is the on-hold period length.

  As suggested by @lbot in his comment, I've reworked these two tips to guide people toward the apps. I'm going to make one other related suggestion in the tip on line 104.

• @mdunninger started a discussion

1. When you send a user an update, the normal action is to set the ticket
  status to `Pending`, which indicates that you are waiting for a reply
  from them. Even though at that point there will be no SLA clock running,
  you might consider setting an expectation with the customer that you'll
  check back with them after an appropriate amount of time to ensure
  continued progress. Should you choose to do that, please consider using
  our [Due Date](https://about.gitlab.com/handbook/support/support-ops/documentation/zendesk_global_apps.html#due-date-picker)
  and [Reminders](https://about.gitlab.com/handbook/support/support-ops/documentation/zendesk_global_apps.html#gitlab-reminders-app)
  apps to help you to meet that commitment.

Related again to the comment from Lee.