Fix "NaN-NaN-NaN" bug in bug/item forms in Firefox

PMT #97819

Firefox was parsing `new Date('Jan. 14 2014');` into an `InvalidDate`,
while Chrome parsed it fine. Note that Firefox parses
`new Date('Jan 14 2014')` (without the dot) correctly.

I considered using moment.js to parse this date, but apparently that's
not moment.js's job. It throws a deprecation warning when you try to use
it to parse dates like that: https://github.com/moment/moment/issues/1407

The way I've solved it is by using datetime.date's `isoformat()`
function to always render this date in a predictable way that should
always be parsed by `new Date();`.
Why are you checking for form existence here?