🚀 [Feature] Backend: Query list all User

[Elweyn]

🚀 Feature

This is a requirement for a number of features:

• Find the Users i want to invite
• What are the Users that are available
• 🌟 [EPIC] [WIP] Suche und Filterung #380 for more details

To-Dos

If we sent a graphql-Query to the backend like this:

{
  users {
    id
    name
    username
  }
}

.. we receive a JSON response with a list of users.