Request to join email should confirm status once it is complete

Description

The "Request to join" email can be sent to a large number of users. There is no quick way for those users to tell if the request has already been completed. Once a recipient of that email has added the user to the group, GitLab should send another email notifying those users that the request has been completed. This can save everyone else on the email some time.

Proposal

After receiving a "Request to join" email, if that user is added to the group, another email should be sent indicating that the request has been completed.

Links / references

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.