Create user activity admin route

[m0ar]

As part of our user research, the product team wants to offer short interviews with active app users.

Create a backend route listing the top (external) users, ranked in a few different categories like nodes recently created, recent activity in the app, etc. For these users, we can check if they opt in to us reaching out and invite them for a chat to understand how they think about the app.

@SamirMamd can chime in on what categories would be useful to sort by.