Geary previously wrapped plain text sigs (i.e. those that did not contain
any HTML elements) in a DIV styled to preserve whitespace. This is all
perfectly fine, except that certain Microsoft products munged the email
for presentation and destroy the whitespace present in the message body,
causing the sig to be mis-rendered.

This works around their shitty software by using BRs instead of the DIV,
also escapes other control chars and sequential whitespace as well when
using a plain text sig. It also stops trying to smart-escape quoted reply
text, since we know it's going to be HTML from the conversation viewer,
meaning smart escaping only needs to be used for sigs.

See issue #29