Fix: show abstract deadline note even when a comment is set

[sarendis56]

What

Conferences that specify both an abstract_deadline and a comment (e.g. S&P, ICSE, USENIX Security, EuroSys, SIGMOD) were silently dropping the abstract deadline note from the displayed NOTE — only the comment was shown.

Why

The render logic only generated the "abstract deadline on …" note when the
timeline entry had no comment:

if item.comment.is_none() {
    item.comment = Some(format!("abstract deadline on {}.", ...));
}

Demonstration

Before the fix:

The "first paper submission deadline" note takes effect, though there is an abstract deadline, it's not shown.

After the fix:

Both are shown, more informative and comply with the intuition.

[mergify]

Welcome to ccf-deadlines!
We're delighted to have you onboard (p≧w≦q)

[mergify]

Great job!
Thank you @sarendis56 for your contribution!