The JavaScript involved wasn't working since we removed the disable-date
attribute in commit 73ff6881.
We're also improving the JavaScript in two ways:
First, we trigger the `change` event immediately, so when the page loads
date fields are disabled when phases are disabled.
And second, we don't remove the selected dates when disabling a phase,
so disabling it and enabling it again will keep the selected values.
1e70a3db02