Remove unused checked assign

* Checked variable isn’t being used anywhere
Lock & hide display date fields if confirmed date

* When the date is confirmed, the display must be exact — users were
confused when they selected a date, the radio option changed but they
were still able to switch it back to one or two month precision
* Instead, switch to exact, lock the form field and hide the one and
two month options
Move release date fields into partial

* Stop repeating fields in new announcement and new date change forms
Don't disable date precision fields

* Disabling fields means their values aren’t passed through
* Presenting a radio button without a choice is confusing, also hide
the radio button when a date is confirmed
Merge pull request #1663 from alphagov/display-date-refinements

Hide display date fields when date is confirmed