Database: FAC Cohort itself could be PRIMARY KEY

[helenzhou6]

Lots of SQL queries need to be changed, since they are based on the fac cohort id.

I do think that the FAC Cohort should be it's separate table though, since when at the filtering stage, when you want to add a FAC Cohort to filter by, there should be an autocomplete input box (that uses SQL query SELECT * FROM fac_cohort to get a list of all the available FAC cohorts - which would be more difficult to do if it wasn't in it's separate table)

Related to refactor list of things to do - issue #59