Use doc element's scrollHeight as ClientWebView's preferred height

This seems to be a bit more accurate than the previous calculation.
Force the doc element's border to zero as well to make using
scrollHeight more reliable.
4 jobs for wip/778025-conversation-height in 7 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #125219
deb-package

00:07:15

passed #125217
fedora

00:07:21

passed #125220
flatpack-package

00:06:56

passed #125218
ubuntu

00:06:45