Add support for relational sorting on queries

[Tworley96]

Currently sorting is only supported for direct fields on a model, I.e. name_asc. Add support for sorting based on rel field -> relation_name_asc.
Note: Currently enums are used for the sorting params, creating this many enums may to much and it might be better to switch to a string representation instead. (I.e. "model.field" instead of an enum for each field of each relationship

[davidism]

In 1.0 I switched from enums to strings, although it still only supports local columns. But filters allow traversing relationships, so we can reuse the same logic.