Date line should be human-readable in mails

[sam (Sam Saffron)]

@joffreyjaffeux / @hugh / @lindsey how do you feel about:

FEATURE: allow customizing default timezone for email.

It:

1. Changes default email format to llll (eg: Tue, May 8, 2018 2:00 AM)
   from 2018-05-08T00:00:00Z UTC which is not nice on the eyes
2. Adds discourse_local_dates_email_timezone which allows configuring
   default timezone in emails
3. Improved help text on site settings (format / timezone)

So in cases of a highly local discourse you would set it to the countries timezone, which leaves emails less confusing.

@Thomas_Rother

OK, in this case, the date line should be transformed into the timezone of the user who receives the notification e-mail

This unfortunately is a bit more complicated, fitting this into the pipeline is not trivial.