Fix date picker for Tasks in Firefox

[cczona]

When starting a new Task, Firefox does not show a date picker. It also does not accept manually entered dates in the following formats. It's unclear what format it wants instead.

• 10/15/2021
• 2021-10-15
• 10/15/2021, 12:00am
• 10/15/2021, 12:00 am
• 10/15/2021, 12:00 AM

In Safari, the date of an existing Task is formatted like the last example. It's known whether that's how it was input via picker, however. A text example would also be useful fallback for situations where the date picker is not available.