feat: add display name column to admin users list

[davwheat]

Supersedes #2998

Progresses #3742

Changes proposed in this pull request:

• Adds a new display name column to the admin users list

Not all users have the same username and display name, so we should show both in the list of users.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[jslirola]

I was going to send my comment when I saw that you have edited the description and you have clarified my question in a moment. I wanted to know if you were making progress on the PR you just added but now it's clear:

#2998

Thank you!

[davwheat]

I'll remove the translation file formatting before I merge.