ConversationWebView: Fix plain text emails sometimes being too wide

Using `whitespace: pre-wrap` to format plain text email sometimes
causes additional width to be allocated by the plain text blocks that
then does not get used due to the constraints on the HTML element.
The allocated space remains however and hence an un-needed horizontal
scrollbar appears.

Using `break-spaces` instead seems to help since it allows breaks after
a space character, leading to the additional space not otherwise being
allocated.
7 jobs for mjog/283-plain-text-whitespace-fixes in 14 minutes and 4 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #952027
fedora

00:06:23

passed #943818
ubuntu

00:07:40

failed #945079
fedora

00:03:20

failed #943868
fedora

00:05:02

failed #943842
fedora

00:04:39

failed #943817
fedora

00:06:22

 
  Test
failed #943819
flatpak
flatpak

 
Name Stage Failure
failed
flatpak Test There has been a missing dependency failure
No job log