add proper aria label to unlock_at date

fixes CNVS-13178

There was no aria label on the text field. There is now

Test Plan
---------------
As a teacher
Given your course has a module
And you are editing that module from the modules page
And you are using a screen reader
When you check the 'lock module until a given date'
And you tab to the 'unlock_at' text field
Then it should read you the label for that text field

Change-Id: I4cce9b038d68917192205b908c1dee0ce6041e3c
Reviewed-on: https://gerrit.instructure.com/35282
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>