Don't add Geary-flavored markdown to plain text messages

Fixes #201
6 jobs for plaintext-markdown in 20 minutes and 34 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #210055
fedora

00:10:27

passed #210056
ubuntu

00:11:27

 
  Test
passed #210057
deb-package

00:08:22

passed #210058
flatpak

00:05:13

 
  Deploy
passed #210059
review

00:00:24

failed #210060
allowed to fail manual
stop_review

00:00:19

 
Name Stage Failure
failed
stop_review Deploy
Using Docker executor with image fedora:29 ...
Pulling docker image fedora:29 ...
Using docker image sha256:26ffec5b4a8ad65083424903b7aa175953329413fe5cc4c0dac6fedbe81f2fbb for fedora:29 ...
Running on runner-d836cf30-project-4116-concurrent-0 via gcc150.bak.milne.osuosl.org...
Cloning repository...
Cloning into '/builds/alex.henrie/geary'...
Checking out 67d81956 as plaintext-markdown...
fatal: reference is not a tree: 67d819565e3af294f0639081d495aa05110a1482
ERROR: Job failed: exit code 1