add custom naming functionality to grading period

closes CNVS-16370

test plan:
-in rails console, create a GradingPeriod and give it a title.
  $ rails c
  $ g = GradingPeriod.new(start_date: Time.zone.now, end_date: 2.days.from_now, weight: 25.0, title: "Custom Name")
-verify you can get the title, change it, and save it.
  $ g.title #should return "Custom Name"
  $ g.title = "New Title!"
  $ g.save #should return true
  $ g.title #should return "New Title!"


Change-Id: I3c1fe8f7f5849fb9550b0006b837e22aeef0abdb
Reviewed-on: https://gerrit.instructure.com/43172
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>