Mentions on new lines after `@gitlab-bot ready` shouldn't be taken in account

Mentions on new lines after @gitlab-bot ready shouldn't be taken in account.

As can be seen in gitlab-org/gitlab!90632 (comment 998022133), currently if you post

@gitlab-bot ready

@user1 hello

it will assign @user1 for review, even though it's not on the same line as the command. We should fix that.