Verbum: allow sharing comments

### Impacted plugin

Jetpack, mu wpcom

### What

Comments on WP.com/JP-connected sites should be sharable

### How

Besides replying and liking a comment, Verbum should include options to share a Comment.

I imagine two flows:

1. Share comment link to Social Media sites: clicking on "Share" would toggle an option to pick from a list of site-owner-defined social media sites
2. Share comment as a new post: clicking on this option for logged in WordPress.com users would open a modal to select a site to post to, and then open a New Post, where the comment's content would be included inside a Quote with a reference to the source.

User request: 8141327-zd-a8c

The issue identifies Verbum in the Jetpack and mu wpcom plugin contexts but names no files, tests, or entry points. Start by locating Verbum's existing reply and like controls, then determine how the two proposed sharing flows fit there. Done would require an agreed implementation of comment-link sharing and sharing a comment as a quoted new post.

Written by the indexing model from the issue text.