Map epics as issues

[gberche-orange]

Currently epics such as https://www.pivotaltracker.com/epic/show/2380443 contain:

• markdown content interesting for "full-text search" and "discovery" use-cases
• interesting cross references to stories, easing navigation.

Suggesting to map epics as issues:

1. fields common to stories mapped in the same way to gh issue fields:
   1. name
   2. description
   3. created_at
   4. epic label as a github label, similar to story labels in #10
2. a deleteable immutable comment which referenced stories associated to the epic. When ever a story gets added/removed from an epic, this would trigger the deletion of the comment, followed by a new comment, enabling to watch changes on epics