Move history contents filtering logic into model.
Filtering for deleted, visible, and ids with ORM should prevent loading unneeded objects into memory (a history with 1000 items and 5 visible will now only cause 5 items to be loaded instead of 1000 just to filter out 5). Simplifies history_contents API logic somewhat, allows easier 'unit' testing (included), and provides a clearer entry point for additional 'showing' additional contents types (read dataset collections).
Merged in jmchilton/galaxy-central-fork-1 (pull request #275)

Move history contents filtering logic into model.