Garbled email addresses in conversation view: Closes bgo#713060

Technically broken (unencoded dot!) "From:" addresses like
=?utf-8?Q?WestLicht.=20Schauplatz=20f=C3=BCr=20Fotografie?= <info@westlicht.com>
were not decoded in Geary's conversation viewer.

Other clients like Gmail and Thunderbird handle the
brokenness, and we should too.

This also closes bgo#714339, "subject contains garbled UTF-8
characters".

https://bugzilla.gnome.org/show_bug.cgi?id=713060
https://bugzilla.gnome.org/show_bug.cgi?id=714339