add asset ordering by tags

[hburgund]

Currently, assets can be ordered in the playlist in a few ways such as by_weight or by_like on a project basis controlled by the project.ordering field which defaults to random.

The desired functionality of ordering by tags is that admins can select a tag and then any assets assigned with that tag will be prioritized in the playlist. If multiple tags exist with the "priority tag", than they will be randomly ordered among themselves, while collectively remaining at the top of the playlist.

This prioritization will be on a project basis, so the prioritized tag should presumably be stored in a new project field i.e. project.prioritized_tag. This field will only be used if project.ordering is set to by_tag and clearly is not a required field and can default to None.

Ordering is done in roundwared/asset_sorters.py, controlled by recording_collection.order_assets() which reads from the project.ordering field.

We should consider #189 alongside this when we devise an approach because they are related and may be able to work in conjunction.