feat: Add CDN support (#1817)

* chore: CSP

* chore: Optionally use CDN for serving images

.env.sample

@@ -10,9 +10,14 @@ DATABASE_URL=postgres://user:pass@localhost:5532/outline
 DATABASE_URL_TEST=postgres://user:pass@localhost:5532/outline-test
 REDIS_URL=redis://localhost:6479
 
+# Must point to the publicly accessible URL for the installation
 URL=http://localhost:3000
 PORT=3000
 
+# Optional. If using a Cloudfront distribution or similar the origin server
+# should be set to the same as URL.
+CDN_URL=
+
 # enforce (auto redirect to) https in production, (optional) default is true.
 # set to false if your SSL is terminated at a loadbalancer, for example
 FORCE_HTTPS=true
@@ -66,4 +71,6 @@ SMTP_REPLY_EMAIL=
 # Custom logo that displays on the authentication screen, scaled to height: 60px
 # TEAM_LOGO=https://example.com/images/logo.png
 
+# See translate.getoutline.com for a list of available language codes and their
+# percentage translated.
 DEFAULT_LANGUAGE=en_US