This class represents "relation" abstraction, which is used for "marking" different entities as related.
So far relations of types "user2content", "user2user" and "content2content" are supported
Good examples of relations are Watches (when user marks certain pages as watchable), Likes (user marks content as "Liked") or
Favourites (content is marked as favourite by user) (all are "user2content" type of relations).
It is very similar to the content properties, but the very important difference is that for given page we're not
interested for all relations, but only if it is related with one particular entity (f.e. to show the page we don't need to
know all the users who "favourited" this page, we just need to know if the current user did).
At the same time we need to fetch all the contentproperties to render the page properly.
So it was decided to create generic relation service to feed and serve "relational" functionality efficiently.