Display the summary authors line only if there is a summary.

The message depends on the number of authors.

1: Summary by <author>:
2: Summary by <first author> & <second author>:
3: Summary by <first author> & <last author> + 1 other:
4+: Summary by <first author> & <last author> + 2 others:

If an existing summary is deleted, the blank summary area should not list an author.

This task was extracted from T98929: C1. "Mark as resolved" for Flow topics