Eager load ListPostsController needed relations

[SychO9]

Part of #2637

Changes proposed in this pull request:
Eager loads the discussion and user.groups relations, leaving discussion.tags and mentionedBy relations until there is a mechanism for extensions to specify needed relations to be eager loaded (which I'll push after this).

Reviewers should focus on:
You can test this by hitting the /api/posts?page[offset]=0&page[limit]=50&include=user endpoint.

Screenshot

Confirmed

• Backend changes: tests are green (run composer test).

[SychO9]

actually, don't merge this yet, I'd like to confirm that these relations are not only needed by specific extensions.