Allow extension of post interaction links

Allow other extensions to add additional interaction links for each Flow
comment. Used by Flow Thanks.

Bug: 61930
Change-Id: Iab4ac2889a3e630a9c98241e4a93c71e4c1bd377
Co-authored-by: Bencmq <bencmqwiki@gmail.com>
Merge "Allow extension of post interaction links"