Add block ordering to models

[bmtcril]

This should allow us to do ordering based on where the block is in the course, and work around the issue of lexical sort on block names when using section:subsection:unit. There will need to be downstream changes in Superset to use it, though.

[Ian2012]

This should allow us to remove all references to the subsection_name_with_location and section_name_with_location.

Is this going to be worked here too?

[bmtcril]

@Ian2012 I think we still want those things, we just want them to sort correctly

[SoryRawyer]

Thanks for adding this!