Moved members migrations to one off job

refs https://github.com/TryGhost/Toolbox/issues/358

- The execution of members migration only ever has to be done once in the lifetime of the Ghost instance. It is slightly slow and blocking process, which slows down instance boot time considerably. Putting the execution into one off job allows to execute migrations only once and save boot time on each consequent instance restart - less resource usage, save the planet!

ghost/core/core/server/services/members/service.js

@@ -148,7 +148,16 @@ module.exports = {
             }
         })();
 
-        await stripeService.migrations.execute();
+        const membersMigrationJobName = 'members-migrations';
+        if (!(await jobsService.hasExecuted(membersMigrationJobName))) {
+            jobsService.addOneOffJob({
+                name: membersMigrationJobName,
+                offloaded: false,
+                job: stripeService.migrations.execute.bind(stripeService.migrations)
+            });
+
+            await jobsService.awaitCompletion(membersMigrationJobName);
+        }
     },
     contentGating: require('./content-gating'),