Add migration to create and update Option rows
Fix Categories

* Make subclasses of CategoryBase, including ControlCategory and
  ControlAssertion, and handle different types of categories the same as
  other polymorphic mappings
Change `Timeboxed` mixin to use Date, not DateTime

* Also add fallback time format
Undefer `context_id` column in general
Merge pull request #620 from reciprocity/feature/fix-options-categories

Fixes around options and categories