Endpoint to retrieve all posts from a given user

[Monstarrrr]

Problem

Currently we're getting all posts from users and filter them on the frontend, even when looking for specific posts, which will get slow.
We need a way to retrieve all posts of a certain property (e.g. from user whose id = 42; with tag = "someTag")

Solution

Create endpoints for those cases:

• Get all posts from user of a given id
• Get all posts with a given tag
• ... will have to think about whether we need more