Route systems and processes through task queue #63056174

To fix [bug 63056174](https://www.pivotaltracker.com/story/show/63056174) (as well as [60289460](https://www.pivotaltracker.com/story/show/60289460), and [63794554](https://www.pivotaltracker.com/story/show/63794554) ).

In doing so, discovered a bug in the lookups of Systems and Processes, which weren't properly searching the whole space of both, as slugs of one of those types must be unique across both.  Fixed by overridding slug lookup method for `SystemsConvertor` to query on the `SystemOrProcess` class (which then permits simplification of the base class).