feat: Untangle some logic

[commonpike]

In general, move the responsibility of post actions to the post model, so the Platform models become simpler.
The post is not a simple child of a platform, it is a join of a platform and a source. Also, models should not have static methods.

• Most importantly, create a PostFactory.resolve(platform,source) instead of static Post.getPost and Platform.getPost or Source.getPost mess. A post does not belong to a platform or a source, but to both
• add a user property to post, because it is neither this.platform.user nor this.source.feed.user
• removing all other static methods from models into their container; because a model is a model
• Move Platform.super.preparePost to post.prepare(); because it didnt add much value and was confusing
• Move plugin.process to Post.prepare - now Platform.preparePost can be empty if all is handled by plugins