Added migration for disallowing bot previews

migrations/20210925171533_bot_preview.js

@@ -0,0 +1,10 @@
+
+exports.up = function(knex) {
+    return knex.schema.table('urls', function (table) {
+        table.boolean('no_preview').defaultTo(false);
+    });
+};
+
+exports.down = function(knex) {
+  
+};