Make default quality a configurable setting

[leonardr]

Currently, when the license source does not provide any information about the relative quality of incoming works, we use a default quality based on the data source. This isn't sustainable because there are tons of data sources, and we can't add a default quality for each of them to the code. Besides which, a library administrator might disagree about the default quality.

Making the default incoming quality a configurable setting will solve both of these problems.