Make action url in guidance context lazy

Otherwise rendering many guidances on request site will create many
autologin URLs via AccessToken objects.

froide/guide/models.py

@@ -78,6 +78,14 @@ def wrapper(self):
     return wrapper
 
 
+class LazyStr:
+    def __init__(self, func):
+        self.func = func
+
+    def __str__(self):
+        return str(self.func())
+
+
 class Guidance(models.Model):
     message = models.ForeignKey(FoiMessage, on_delete=models.CASCADE)
     action = models.ForeignKey(Action, null=True, blank=True, on_delete=models.CASCADE)
@@ -123,10 +131,14 @@ def get_context(self):
             "user": user,
             "name": user.get_full_name(),
             "message": self.message,
-            "action_url": "{}-guidance".format(self.message.get_autologin_url()),
+            "action_url": LazyStr(
+                lambda: "{}-guidance".format(self.message.get_autologin_url())
+            ),
         }
         if self.action and self.action.letter_template_id:
-            ctx["action_url"] = user.get_autologin_url(self.get_letter_url())
+            ctx["action_url"] = LazyStr(
+                lambda: user.get_autologin_url(self.get_letter_url())
+            )
         return ctx
 
     @render_with_context