Improve UX on bulk filter/assigning

We are seeing a number of people accidentally bulk assigning people or labels to issues recently. Since this is happening within our own team, I think we should understand how this is happening so that we can improve the UX. For example:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/29425#note_25310225
• https://gitlab.com/gitlab-com/infrastructure/issues/1319

@collen, @maratkalibek, could you explain what happened and what might have been confusing?