Avoid unnecessary series type queries

[jyggen]

The access to series_type.id causes the related series type to be queried every time an issue or series is turned into its string representation, which can results in dozens of additional queries on some views. Since only id is accessed, __str__ can be optimized into using series_type_id instead, which avoids the additional query altogether.

[bpepple]

Looks good. This change should avoid any unnecessary DB queries every time these models are stringified (e.g., in admin lists, logging, API serialization, etc.) where the related object is not already cached in the ORM. Thx!