[Feature] Line-based comment/review feature

**Is your feature request related to a problem? Please describe.**

When working in teams we are using github and dropbox paper to do line by line review/comments of stuff we draft in netlifycms. this seems like it should be part of netlifycms.

**Describe the solution you'd like**

github review process, but inside netlifycms

**Describe alternatives you've considered**

none

The issue names no files, tests, or entry points. Start by mapping the existing content-editing and review workflow, then clarify the scope of GitHub-style line comments inside Netlify CMS; done requires an agreed interaction model and implementation boundaries.

Written by the indexing model from the issue text.