Migrate from REST API to GraphQL API

[chshersh]

We can attempt to migrate to a newer API after the following issue in github-graphql is implemented:

• Minimal requirements to support hit-on github-graphql#25

Since we will write a lot of custom queries for GitHub, I propose the following new module structure:

• Replace Hit.Issue with Hit.GitHub.Issue + Hit.GitHub.PullRequest + Hit.GitHub.Milestone + Hit.GitHub.Fork
• Each module will contain queries, types with FromJSON instances and functions to fetch relevant information

Once the migration is done, we need to test that our code still works as expected. User-stories to test (feel free to add more):

• Fetch single issue and display all info
• Fetch all issues
• Fetch issue and filter by login
• Fetch issue and filter by milestone
• Fetch issue and filter by both login and milestone
• Fetch only issue title
• Fetch the latest milestone
• Fetch all milestones
• Assign to the existing issue
• Create a new issue with the assignment
• Fork
• Create a PR