Core: extend REST API with fetchComposite (#659).

Core REST API: the topic's and association's "get related topics" request takes 2 optional query parameters (boolean): `fetch_composite` and `fetch_relating_composite`. As usual they respectively indicate weather the child topics should be included in the response.

1. The related topic's child topics (`fetch_composite`).
2. The child topics of the association that connects the related topic (`fetch_relating_composite`).

Default value for both parameters is `false`.

{{{
GET /core/topic/{id}/related_topics?...&fetch_composite=true&fetch_relating_composite=true

GET /core/association/{id}/related_topics?...&fetch_composite=true&fetch_relating_composite=true
}}}

See #659.