Refactor: Separated "Other Videos" section into a dedicated component/service (#969)

Refactor: Separated "Other Videos" section into a dedicated component/service (#969)

* Separated "Other Videos" section into a dedicated component/service

I'm currently working on some proof-of-concepts for recommendation
providers that could work with PeerTube to provide useful video
suggestions to the user.

As a first step, I want to have great clarity about how PeerTube,
itself, will surface these videos to the user.

With this branch, I'm refactoring the "recommendations" to make it
easier to swap out different recommender implementations quickly.

Stop recommender from including the video that's being watched.

Ensure always 5 recommendations

* Treat recommendations as a stream of values, rather than a single async value.

* Prioritize readability over HTTP response size early-optimization.

* Simplify pipe