Migrate mass-assignment security to strong_parameters

Ahead of the upgrade to Rails 4, this change removes the use of
attr_accessible and attr_protected as mass assignment protection
mechanisms in favour of whitelisting input attributes at the
controller level.

See http://blog.markusproject.org/?p=5588 for more details.
Handle submission rule changes correctly in assignment editing

Resolves GH-1622.
Fix section due date duplication

They were not being properly whitelisted previously, and the
workaround did not check if the due dates already existed before
creating new due dates.

This fix allows Rails to simply Do The Right Thing.
Really delete old section due dates when they get turned off

And fix broken tests which were providing false failures.
Merge pull request #1609 from ferrous26/strong_params

Migrate mass-assignment security to strong_parameters