Datetime widget fixes and improvements

[martinjagodic]

Targeting all open issues with the label area: extensions/widgets/datetime. Hopefully, I get it right this time.

• don't display the current date by default or when the field is empty (#3679) - potentially breaking change
• add default: '{{now}}' option to enable the current behavior of displaying the current time (using the specified format)
• add UTC indicator when picker_utc: true
• improve how Z in format and picker_utc: true work together
• reorder format importance: if format is set, date_format and time_format (if strings) are ignored (#7250)

[demshy]

Looks like you also fixed this one: https://github.com/decaporg/decap-cms/issues/7212