Standardize how dates are displayed in interleaved view headers

We want to update how dates are displayed in Search results, as well as in 'Starred messages'.

Currently the date is shown on every single message header, but the way we display the year presents a situation where you can have an identical date in two different formats: The first message sent in a new year will include the year, and then the next one will include the date but not the year. This can be confusing or look like a bug as noted on CZO.

Instead, we should continue always show year for past years, and show never a year for the current year. Technically: we should stop passing the cross-message year_changed into the header date when always_display_date is on.

CZO thread: #issues > Date stamps in search results