DB queries on admin_init.

The code is currently running a query to create a DB table if doesn't already exist on `admin_init`.

https://github.com/humanmade/protected-embeds/blob/32f1e61af00d8ab0e86764b29e87077956911515/protected-embeds.php#L14

Due to how often `admin_init` is triggered this is potentially creating a large number of unnecessary queries, for instance we have literally seen the `create_database_table()` function called millions of times in a single day on one of our big sites.

It would be more efficient to check the table exists on activation and upgrade rather than on every admin action.

Please give me a shout if you would like me to work on this with you.

Thanks.