fix unlocked_at field when editing a module

fixes CNVS-13180

When you edit a module item and click to add an unlocked_at date it was
acting a little weird. It would "persist" after you added an
unlocked_at date when editing another unlocked_at date in a different
module as well as it would loose the previously selected unlocked_at
date when checking/unchecking the checkbox. This fixes all of that.

Test Plan
-------------------
As a teacher
Given you have 2 modules that don't have a unlocked_at due date set
And you are on the modules page
When you click the gear icon and edit a module
And you check the 'lock module until a given date' checkbox
And you select an unlocked_at date
And you uncheck the 'lock module until a given date' checkbox
And you check 'lock module until a given date' again
Then you should see the unlocked_at date field should have the
previously selected due date. It should not be 'lost'
---------------------------------
Given you have 2 modules that don't have a unlocked_at due date set
And you are on the modules page
When you click the gear icon and edit a module
And you check the 'lock module until a given date' checkbox
And you close the module edit dialog window
And you edit a different module with no unlocked_at due date set
Then you should see that module's 'lock module until a given date'
checkbox is unchecked
And you don't see the 'unlocked_at' field.
---------------------------------------------------
Given you have a module with a due date
And you are on the modules page
When you click the gear icon and edit that module
Then you should see the 'lock module until a given date' checkbox
checked
And there should be a unlocked_at date set in the field

Change-Id: Ic70ebefaa2cadb684266cdd442b98751a6739cf7
Reviewed-on: https://gerrit.instructure.com/35278
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>