add number to assessments

This is to prepare for ordering assessments by their number, so we
can custom set the order. When a new assessment is created, the
number will default to one more than the last assessment for that
cohort.
add default scope to ensure assessments are always ordered by number
add drag and drop to change order of assessments
round out persmissions for assessments
cleanup assessment sorting
remove section and url attributes from assessments
fix typos and indentation
Merge pull request #69 from epicodus/assessment-order

Assessment order
add ability to create a new cohort
add datepickers to cohort#new page
style cohorts#new page
restructure new cohort logic
Merge pull request #70 from epicodus/creating-cohorts

Creating cohorts
create new BalancedEvent for every callback
add status to payments
add correct starting status to new payments
update payment status when callback occurs