Set up Users table and get user info from github API

As a user of gh-backup
I want to be able to identify who has written the versions of comments
So that I know who to hold responsible for them

- [x] Set up a Users table which includes fields for the user's login details (email?) and the user's avatar url
- [x] Link the User table with the Issue/Comments table
- [x] Add the relevant user info to the issue/comments/users table when a change is made
- [x] Include the avatar url or user_id in order to generate this