`reply-reviewers` as an endpoint on reviewers input within reply dialog,
which provides two slots:
1. `right` which will appear on the right of the input
2. `below` which under the input
Add two events in reply dialog to support add / remove reviewers.
Added a sample of how to use these two endpoints.
Screenshot: https://imgur.com/P4VluCP
Change-Id: Ie3ce34f004d347ccd5343a0ef820efa2297df89e