settings: Add option in settings_checkbox to show some text in parens.

There are cases where we add some text in italics inside parens along
with the label in checkbox settings.

This commit adds option to settings_checkbox template so that we can
directly use it and not require to duplicate the settings_checkbox
code and thus we now use settings_checkbox template for
enable_read_receipts setting.

static/js/admin.js

@@ -53,6 +53,10 @@ const admin_settings_label = {
         defaultMessage: "Language for automated messages and invitation emails",
     }),
     realm_allow_message_editing: $t({defaultMessage: "Allow message editing"}),
+    realm_enable_read_receipts: $t({defaultMessage: "Enable read receipts"}),
+    realm_enable_read_receipts_parens_text: $t({
+        defaultMessage: "Users can always disable their personal read receipts.",
+    }),
 };
 
 function insert_tip_box() {