Simplify / unify api a little

[kesla]

I think that we should remove the url field when possible, for example when we parse instgram we return both the url and the id - but we can create the url from the id.

Similarly we return both the twitter url - but also the twitter id and the twitter username.

Oh, we should probably also rename youtubeId in youtube to id.

@iefserge @ellell WDYT?