[Refactor] Passwort Hash/Set (#14282)

* move SaltGeneration into HashPasswort and rename it to what it does

* Migration: Where Password is Valid with Empty String delete it

* prohibit empty password hash

* let SetPassword("") unset pwd stuff

routers/admin/users.go

@@ -267,7 +267,10 @@ func EditUserPost(ctx *context.Context, form auth.AdminEditUserForm) {
 			ctx.ServerError("UpdateUser", err)
 			return
 		}
-		u.HashPassword(form.Password)
+		if err = u.SetPassword(form.Password); err != nil {
+			ctx.InternalServerError(err)
+			return
+		}
 	}
 
 	if len(form.UserName) != 0 && u.Name != form.UserName {