Initial work on a tags manager.

Rest of UsesTagsMixin needs to be copied over similarly and deprecated, following that I guess all references need to be updated and then the mixin eliminated all together I guess (exhausting :)).
Refactor generic controller mixin code for security out for reuse in managers.
Refactor generic controller mixin code for loading objects into managers.
Introduce LDDA manager stub.

Use it to eliminate awful controller dependencies (and associated circular dependency errors when controllers are not loaded before everything else) in galaxy.dataset_collections.
Make dataset collections service a manager.

It would hve been arcitected this way from the beginning but my initial work on it predated Carl's managers work and it had those pesky controller dependencies.
Allow uuid values in to_dict.
Introduce workflow recover state functionality.
Work on workflow module testing.