Username autocompletion (non-members)

Problem to solve

When discussing in an issue, only the usernames of project or group members are autocompleted. It would be nice if it was possible to have autocompletion for non-members too.

Intended users

Everyone

Proposal

Extend the look-up-pool for autocompletion using the @ key to all users. If it's too heavy for large installations (i.e. user-bases) I'd propose to either have an option to activate it, or only add already mentioned names to the autocompletion list, so if one username has been been typed once in a discussion, it is recognised in subsequent comments.

Permissions and Security

I am not sure if it's OK to reveal all possible names.

Documentation

Testing

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

Links / references