Set white background for mails in Outlook

Outlook seems to have problems in some versions to interpret the short color code.
Using the long form version fixes that and makes mails look nicer in Outlook.

Signed-off-by: Martin Schurz <Martin.Schurz@telekom.de>

.stylelintrc

@@ -6,5 +6,13 @@
      "ee/app/assets/stylesheets/startup/startup-*.scss",
      "app/assets/stylesheets/highlight/themes/*.scss",
      "app/assets/stylesheets/lazy_bundles/cropper.css"
+   ],
+   "overrides": [
+    {
+      "files": ["app/assets/stylesheets/mailer.scss"],
+      "rules": {
+        "color-hex-length": "long"
+      }
+    }
    ]
 }

app/assets/stylesheets/mailer.scss

@@ -3,12 +3,12 @@
 // Do not use 3-letter hex codes, bgcolor vs css background-color is problematic in emails
 //
 $mailer-font: 'Helvetica Neue', Helvetica, Arial, sans-serif;
-$mailer-text-color: #333;
+$mailer-text-color: #333333;
 $mailer-bg-color: #fafafa;
 $mailer-link-color: #3777b0;
-$mailer-link-muted-color: #333;
+$mailer-link-muted-color: #333333;
 $mailer-line-cell-bg-color: #6b4fbb;
-$mailer-wrapper-cell-bg-color: #fff;
+$mailer-wrapper-cell-bg-color: #ffffff;
 $mailer-wrapper-cell-border-color: #ededed;
 $mailer-header-footer-text-color: #5c5c5c;
 $full-width: 640px;