Add a warning when assigning or pinging somebody outside of the project

Problem to solve

With similar usernames issues or merge requests can be assigned to the wrong person or the wrong person can be pinged. This is especially a problem on instances with many users such as gitlab.com.

Further details

This idea comes from https://gitlab.slack.com/archives/C0SFP840G/p1537783310000100.

Proposal

Let a user know if the person who they assign or ping is not a member of the project.

What does success look like, and how can we measure that?

A warning message is displayed before pinging / assigning a random stranger.