Enh: UI - missing buttons if no emails are displayed

The buttons to (un)subscribe, post and maintain favourites are not displayed unless there are some emails that fit the criteria (e.g. last month).

This avoids dealing with missing or inaccessible lists, but it makes some actions harder.
In particular removing a favourite or unsubscribing from list that is no longer active.

The code should be able to check if the user has access to at least some of the messages on the list and display the buttons if so.

Start by tracing the UI code that decides whether messages are displayed and whether the subscribe, post, and favourites buttons are rendered. Check how access to messages on a list is determined; done means those buttons remain available when the user can access at least some messages, even if none match the current criteria.

Written by the indexing model from the issue text.