Allow changing date and time format

The format strings are hardcoded in the templates:

• https://gitlab.com/mailman/hyperkitty/-/blob/master/hyperkitty/templates/hyperkitty/fragments/overview_threads.html#L17
• https://gitlab.com/mailman/hyperkitty/-/blob/master/hyperkitty/templates/hyperkitty/messages/message.html#L32-40
• https://gitlab.com/mailman/hyperkitty/-/blob/master/hyperkitty/templates/hyperkitty/threads/summary_thread_large.html#L38-41 (timesince really isn't very useful for years old archives)

Over here in remote lands of europe we prefer 24H format for time, it may be different for even different cultures :P