Content model cleanup for renditions

[cmlewis]

There's a lot going on with rendition types and properties and aspects in the OC content model. Some of them we use, others were added in hopes of switching how we do renditions in HPI, but it's just not feasible. Tasks to clean up include:

• [x] Remove tsg:tsgRendition type (not in use)
• [x] Rename hpi:rendition TYPE to something like hpi:preview to distinguish from the old hpi:rendition and new tsg:rendition PROPERTIES (used for real renditions for asset thumbnails, video previews, etc.)
• [x] If possible, clean up any old hardcoded references to the hpi:rendition PROPERTY so core will use tsg:rendition (or a configured rendition property) as much as possible

_Some related issues that might be good to knock out at the same time:_

• https://github.com/tsgrp/OpenContent/issues/19
• https://github.com/tsgrp/OpenContent/issues/30
• https://github.com/tsgrp/OpenContent/issues/31

[cmlewis]

Whoops, this was finished on July 21, SVN rev 9259.