[IMP] digest: add private methods to manage digest members

Purpose is to ease digest management and testing. Public methods still work
as before this commit, simply calling the private implementation. New private
methods allows to better manipulate subscribed users. Those will be used
notably as shortcuts in tests.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onbarding and usage improvement)

Part-of: odoo/odoo#79653

addons/digest/models/digest.py

@@ -87,11 +87,21 @@ class Digest(models.Model):
 
     def action_subscribe(self):
         if self.env.user.has_group('base.group_user') and self.env.user not in self.user_ids:
-            self.sudo().user_ids |= self.env.user
+            self._action_subscribe_users(self.env.user)
+
+    def _action_subscribe_users(self, users):
+        """ Private method to manage subscriptions. Done as sudo() to speedup
+        computation and avoid ACLs issues. """
+        self.sudo().user_ids |= users
 
     def action_unsubcribe(self):
         if self.env.user.has_group('base.group_user') and self.env.user in self.user_ids:
-            self.sudo().user_ids -= self.env.user
+            self._action_unsubscribe_users(self.env.user)
+
+    def _action_unsubscribe_users(self, users):
+        """ Private method to manage subscriptions. Done as sudo() to speedup
+        computation and avoid ACLs issues. """
+        self.sudo().user_ids -= users
 
     def action_activate(self):
         self.state = 'activated'