Enh: store/show original Date:

The 'date' field is derived from the Date: header, and is currently stored in UTC.
This loses some information (though the original value is available in the source).
It might be useful to store the raw date string as well.
At present the date is shown in the GUI in the user's timezone using a non-standard format, e.g.

```Date: 2016-11-04 16:00 (-0000)```

It might be useful to show the date as follows:

```Date: 2016-11-04 16:00 -0000 (Fri, 4 Nov 2016 12:00:48 -0400)```

Start by tracing how the Date: header is parsed and stored, then follow the GUI code that renders the date in the user's timezone. Done means the raw date value is preserved and the GUI displays both the localized date and the original header date, including its timezone offset.

Written by the indexing model from the issue text.