Epic: Define (Ecto) PostgreSQL Database Schema

All the "Meta Data" (_anything that is not an issue comment, which is Markdown/"Rich Text"_)
of issues will be stored in a PostgreSQL Database.

With reference to the GitHub API for `users`, `issues` and `comments`,
we need to create the Ecto (Database) Schema for the three DB tables.
We are going to "normalise" the data and let Ecto construct the queries.

+ [ ] Users: https://developer.github.com/v3/users
+ [x] Issues: https://developer.github.com/v3/issues
+ [x] Comments: https://developer.github.com/v3/issues/comments

Our aim is to match the _field names_ in the GitHub API _exactly_
so we keep an _exact_ copy the data that people can _easily_ download
if they want to have a Local Copy. (_don't worry, we aren't building the download feature **yet**!_)

Start with the linked GitHub API documentation for users, issues, and issue comments, then inspect the repository to find its Ecto schema and migration entry points. Done means defining normalized database tables for all three resources while preserving the GitHub API field names exactly; the checklist shows issues and comments as complete but users still outstanding.

Written by the indexing model from the issue text.