Public memos should be visible also to authenticated users

[benel]

Reproduction scenario :

1. Go to Cassandre.
2. If you are logged in, log out.
3. Open a public diary and a public memo.
4. Check that the content is visible.
5. Create a new account (or log in to an account with no special rights on the memo).
6. Open a public diary. "Nothing to be shown" is displayed.

Tested version: current master (8b39dc4).