In 07ecbb5a3b we ensure the mentions in a group's activity page worked properly but we missed adding proper support for infinite loading.
The client is using the `before` parameter instead of the `before_post_id` to do the pagination.
This adds support for `before` as well as some tests to ensure it doesn't regress.
I also added tests to the group's activity posts as well since those were missing.
Finally I deleted some unused code (`group.messages_for`) which is not used anymore.
Context - https://meta.discourse.org/t/-/308044/9