new migration prototype
new migration prototype

- change hook from `init` to `admin_init` → drastically decreases
chances of multiple migrations running at the same time (bonus: one
less option-query in frontend)
- multisites must be handled explicitly now, otherwise the admin has to
visit all of them — and in that time the frontend might be failing

katrinleinweber/podlove-publisher,noplanman/podlove-publisher,noplanman/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,katrinleinweber/podlove-publisher,katrinleinweber/podlove-publisher,katrinleinweber/podlove-publisher,katrinleinweber/podlove-publisher,noplanman/podlove-publisher,noplanman/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher