Change the multisite_account.json API to from GET to POST

[tanius]

Since the multisite_account.json API results in status changes, it should use POST and not GET. We attempted this before but there was some problem. We have since enabled working CORS requests in Discourse, so should be able to get this to work now. (I don't really remember if CORS was the problem back then, though.)

This will also require changes to the API clients or they would stop working. See our API client registry for which clients are affected.