models: Rename Realm.show_digest_email field.

This renames Realm.show_digest_email field to
digest_emails_enabled, for greater clarity as to what it does
just from seeing the setting name, without having to look it up.

Fixes part of #10042.

static/js/settings_notifications.js

@@ -52,7 +52,7 @@ function change_notification_setting(setting, setting_data, status_element) {
 }
 
 exports.set_up = function () {
-    if (!page_params.realm_show_digest_email) {
+    if (!page_params.realm_digest_emails_enabled) {
         $("#digest_container").hide();
     }