[feature/api] Merged topic repo with forum repo

It was confusing that the topic repository and controller returned a
list of topics in a given forum and not a topic with posts. Therefore
the topic listing has been moved to the forum repository to make it
clearer and also to follow the routing pattern better.

PHPBB3-11595
[feature/api] Added API to retrieve the posts in a topic

Added necessary functions to be able to retrieve posts in a given
topic.

PHPBB3-11595