Refactor FeedSource, FeedTransformRules and FeedRetrievalMethod

[binh-dam-ibigroup]

Original comment by @evansiroky: #371 (review)

I think we may want to rethink whether we should have the transform rules be a list within the FeedSource object. I think it deserves its own mongo collection because it is probably unnecessary to fetch all transformation data whenever a feed source is fetched. Also, it might also make sense to combine the FeedTransformRules, FeedRetrievalMethod and FeedTransformation classes together into a single FeedTransformation class. These design decisions were made prior to this PR, but I think moving forward, we should seriously consider refactoring this and avoid making additional lists within model classes unless a very small amount of data is expected within these lists.