Fix inconsitent db schema
Add UserEnqueuedSubjects denormalization

Add after_create and after_save callbacks to Classification
model to manage denormalization
Add queued sort to subjects controller
Refactor classifications model

Use better boolean method naming, remove state enum
Remove UserEnqueuedSubject model when no more subject_ids
Add SubjectSelector class
Update for strong params configuration change
Fix counting error
Initial Work towards a better subject queue
Rename UserEnqueuedSubject to UserSubjectQueue