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**!_)