[feature] New user sign-up via web page (#2796)

* [feature] User sign-up form and admin notifs

* add chosen + filtered languages to migration

* remove stray comment

* chosen languages schmosen schmanguages

* proper error on local account missing

internal/web/robots.go

@@ -82,6 +82,7 @@ Disallow: /oauth/
 Disallow: /check_your_email
 Disallow: /wait_for_approval
 Disallow: /account_disabled
+Disallow: /signup
 
 # Well-known endpoints.
 Disallow: /.well-known/