chat: Disable asynchronous image loading

The problem is currently that not all images are displayed when
asynchronous loading is on.
8 jobs from master in 4 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2250
clazy

00:00:44

passed #2251
qmllint

00:00:36

 
  Build
passed #2255
archlinux

00:00:41

passed #2254
debian-sid

00:01:06

passed #2253
neon-bionic

00:01:07

passed #2252
ubuntu-trusty

00:01:04

 
  Deploy
passed #2257
click-xenial

00:02:16

passed #2256
linux-appimage

00:02:18