Subjects (prototype)

- Users have the option of selecting existing subject or adding new
subject when they add a unit
- subject tag will not show up until its status is changed from
'pending' to 'confirmed'
- added a subjects controller and subject view to test if subjects were
showing up properly
-- subject view shows primary material of units tagged with that subject
- skilljoy database removed from schema and re-inserted as individual
tables
- subjects table updated to include 'status' column
- units controller modified to also send array of subject tags to unit
view