Reorganize model/controller helper code

[begedin]

Taken from discussion in #139

It looks like the functions we have there would fit into something like CodeCorps.Slugs and CodeCorps.Filtering more so than the current CodeCorps.ControllerHelpers and CodeCorps.ModelHelpers naming/organizational scheme.

Actual naming and organization is open for discussion, of course.

[joshsmith]

Some progress was made on this in #332 but could be refined further.

[joshsmith]

In particular would like to move most of what's in the model helpers into query helpers, but eventually replace even most of those with filters that primarily use filtering in ja_resource or something.