remove unneeded submission views
reconfigure routes for submissions

Originally we wanted to have a single resource for submission nested
within assessments so the url would look something like this:
`/assessments/3/submission`. This was rendered unnecessary when we
moved the feedback for an assessment on to the assessment page.
remove unneeded controller actions for assessments and requirements
fix bug that would show multiple error messages if a review was miscreated
remove old factory for assessment_requirements

This must have been pulled in when cherry picking commits from the
epissessments app. Removing now.
move factories into factories.rb

Using model generators would also create a separate factory file
for that model. For consistency's sake, I've moved all the factories
created this way into the file where all the other factories live.
Someday they should be refactored into their own files, but it is
not this day.
add uniqueness validation to ensure users can only have one submission per assessment