Optimise Fragment type for network performance

[stefankreitmayer]

Current solution

In the current Elm implementation, each Fragment contains the OER data it pertains to.

type alias Fragment =
  { oer : Oer
  , start : Float -- 0 to 1
  , length : Float -- 0 to 1
  }

This implementation is neat in the sense that it keeps the frontend code clean and simple. However, it makes the Fragment type less suitable for use in client-server communication, as Oer data can be large.

Better solution

A more network-friendly solution would avoid storing the entire OER in the Fragment and instead use only its URL.

type alias Fragment =
  { url : String
  , start : Float -- 0 to 1
  , length : Float -- 0 to 1
  }

Pro

• better network performance when dealing with large OERs and/or many fragment-wise recommendations

Con

• increased complexity in the frontend code, as Fragments and OERs must be loaded and managed separately.

[stefankreitmayer]

This was solved as part of #42