org settings: Change message_edit settings to dropdown.

This coverts the "checkbox" for `realm_allow_message_editing`  and
"input" for `realm_message_content_edit_limit_seconds` into a
dropdown with the option for custom time limit option.

static/js/settings_ui.js

@@ -18,6 +18,7 @@ function _initialize() {
     };
 
     settings_bots.setup_bot_creation_policy_values();
+    settings_org.setup_msg_edit_limit_dropdown_values();
 }
 
 exports.initialize = function () {