Removing the Chat Notifications user from the permissions of a space doesn't have a way to add it back

Issue Summary

When playing around space permissions, there is a user called Chat Notifications that is responsible for the Slack integration. If we remove this user from the space permissions, there is no way via UI to add-it back.

Steps to Reproduce

1. On any space, go to Space Settings > Permissions;
2. Click over the Edit Permissions button;
3. On the Chat Notifications user, deselect all the checkboxes and save it;
4. Edit the permissions again and try to add the Chat Notifications user.

Expected Results

As soon as you start to type, the Chat Notifications user is shown in the suggested users list.

Actual Results

No suggestion is shown.

Workaround

1. Create a new group on your Site Administration
2. Either add the Chat Notification user (Account ID: 5b70c8b80fd0ac05d389f5e9)  to the group using the Add member to group REST API endpoint or raise a support ticket asking to our folks to add the Chat Notifications user in this group.
3. Add the group in the space permissions, so the Chat Notifications user will have permissions in the space again.