Partner logos stored in sessions may be out of date

When visiting a random page, a variable is added to a user's session that contains the sequence in which partner logos are displayed in the header. If partners are then updated (added / modified / deleted), the sequence in the session no longer matches reality. However, there is no way to invalidate this.

It can be solved by caching some sort of checksum over the existing partners (something that does not hit the database on every request!) and comparing this to the value in the user's session. Calling save() on a partner would then need to invalidate this cache and recompute the checksum.

Notably, the @cached_property decorator does not apply here, as it is recomputed for every new instance (and thus does not persist across requests).