Disable user-visible UI for editing messages, for MIT.

This doesn't actually prevent a user from making the API call into our
servers to actually go and edit a message, so this isn't a bulletproof
solution for realms where messages ABSOLUTELY MUST NOT be edited.

(imported from commit 5bf043a201e2952189b45f93b8c5ca7648f6aee7)

static/js/feature_flags.js

@@ -25,6 +25,7 @@ exports.log_send_times = false;
 exports.collect_send_times = false;
 
 // Permanent realm-specific stuff:
+exports.disable_message_editing = _.contains(['mit.edu'], page_params.domain);
 
 exports.twenty_four_hour_time = internal_24_hour_people || iceland;