Remove deactivated user from group.

Description:

Deactivated users are no longer shown in the member list of a group, but when they are reactivated, they are automatically in this group again. see: #18711 https://github.com/RocketChat/Rocket.Chat/issues/18711#issuecomment-685894273
As far as I can see, there is no way to remove a deactivated user from a group. One might want to do this, if the content of the group is only meant for a very limited set of users and there is the possibility of deactivated users being reactivated in the future.

Steps to reproduce:

1. create a group with multiple users.
2. deactivate one of these users.
3. search the deactivated user in the member list.

Expected behavior:

It should be possible to remove deactivated users from groups, to prevent sensitive information being leaked to reactivated users.

Actual behavior:

Since deactivated users don't show up in the member list (even with the search function), one cannot remove them (as far as I can tell).

Server Setup Information:

• Version of Rocket.Chat Server: 3.11.2

Client Setup Information

• Desktop App or Browser Version: Desktop App 2.17.11
• Operating System: GNU/Linux

Reproduce the group-membership flow using the listed steps, then read the related discussion in issue #18711. Trace how deactivated users are hidden and how group membership is restored on reactivation; done means a deactivated user can be removed from a group and does not automatically regain access after reactivation.