Update logic for calculating student response rate. Remove references

to survey table.  We no longer check or keep track of the survey type.
Instead we look in the database to see if a survey item has at least 10
responses.  If it does, that survey item was presented to the respondent
and we count it, and all responses when calculating the response rate.

Remove response rate timestamp from caching logic because we no longer
add the response rate to the database. All response rates are calculated
on the fly

Update three_b_two scraper to use teacher only numbers

swap over to using https://profiles.doe.mass.edu/statereport/gradesubjectstaffing.aspx as the source of staffing information

app/services/staffing_loader.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-# TODO
 require 'csv'
 
 class StaffingLoader