/unassign command does not work as expected with multiple assignees

Assigning

CE: /assign @user Assign user

EES: /assign @user1 @user2 Assign user(s)

Unassigning

CE: /unassign @user Remove assignee

CE /unassign (Unassigns yourself)

EES: /unassign @user1 @user2 or @all-assignees Remove assignee(s)

EES /unassign (Unassigns yourself)

System notes

Specified in description of https://gitlab.com/gitlab-org/gitlab-ee/issues/1904.

Original description

I have an issue assigned to multiple assignees, @user-a and @user-b
I used /unassign @user-b to remove @user-b from the issue.
I expected the assignment list to still have @user-a as an assignee, but the slash command had the effect of removing all assignees.

Design

Command	Action
/assign @user1 @user2	Assign user(s)
/unassign @user1 @user2	Remove all or specific assignee(s)
/reassign @user1 @user2	Reassign to user(s)

We don't need a special command to assign to the current user; we don't currently have this for non-multiple-assignee /assign either. When you type /ass[TAB], it will automatically autocomplete to /assign @, which will show the dropdown with the current user's username high up in the list.

This is consistent with the current slash commands for labels:

Command	Action
/label ~foo ~"bar baz"	Add label(s)
/unlabel ~foo ~"bar baz"	Remove all or specific label(s)
/relabel ~foo ~"bar baz"	Replace all label(s)

And mostly consistent with non-multiple-assignee slash commands in CE, which is important for people upgrading, with the only difference being that /assign adds to the current assignee(s) instead of overwriting them, but I think that makes sense:

Command	Action
/assign @username	Assign
/unassign	Remove assignee