POSTs to wikibase API endpoints that create a temp user now should return a tempuserredirect URL now (in addition to tempusercreated, which still contains the username)

To handle this, we need to :

• set returnto to the current page title when sending the POST
• optionally returntoquery or returntoanchor when sending the POST to let the code realize, once the user was redirected back to commons, that a temp user was created and it should show some sort of message about it (and, if necessary, select the "structured data" tab)

Then if the tempuserredirect is non-null in the response to the POST, we need to redirect to that