GRN2-xx: Added SAFE_HOSTS env variable to block unknown hosts (#1543)

* Added SAFE_HOSTS env variable to block unknown hosts

* Update sample.env

sample.env

@@ -16,6 +16,13 @@ SECRET_KEY_BASE=
 BIGBLUEBUTTON_ENDPOINT=
 BIGBLUEBUTTON_SECRET=
 
+# The hostname that the application is accessible from. 
+#
+# Used to protect against various HTTP header attacks
+# Should be in the form of "domain.com"
+# 
+SAFE_HOSTS=
+
 # Google Login Provider (optional)
 #
 # For in-depth steps on setting up a Google Login Provider, see: