Add stylelint to CIs

.buildkite/pipeline.yaml

@@ -3,6 +3,7 @@ steps:
     command:
       - "yarn install"
       - "yarn lintwithexclusions"
+      - "yarn stylelint"
     plugins:
       - docker#v3.0.1:
           image: "node:10"

jenkins.sh

@@ -29,6 +29,9 @@ yarn lintall -f checkstyle -o eslint.xml || true
 # re-run the linter, excluding any files known to have errors or warnings.
 yarn lintwithexclusions
 
+# lint styles
+yarn stylelint
+
 # delete the old tarball, if it exists
 rm -f matrix-react-sdk-*.tgz