Don't ignore first action when assign and unassign quick actions are used in the same comment

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7478

This patch fixes the following scenarios when multiple assignees are available (on issues, on EE Starter):

# Assignees: @user1 @user2

# Submit single comment with:
/assign @user3
/unassign @user2

# Assignees before this patch: @user1 -- wrong :( The first action was ignored
# Assignees after this patch: @user1 @user3 -- right! :D

# Assignees: @user1 @user2

# Submit single comment with:
/unassign @user2
/assign @user3

# Assignees before this patch: @user1 @user2 @user3 -- wrong :( The first action was ignored
# Assignees after this patch: @user1 @user3 -- right! :D