Migrate coala-json from GitLab to GitHub

One of this year's GSoC projects is Result Reporter Tool (see https://summerofcode.withgoogle.com/projects/#6527640138153984 and http://projects.coala.io/#/projects?project=result_reporter_tool&lang=en).

Currently, we have a repo called 'coala-json' on GitLab, https://gitlab.com/coala/coala-json, which could be used as a bed for the Result Reporter Tool project. However, as this project involves some CI integration, e.g. CircleCI, it is better to have the repository resided on GitHub (GitHub supports CircleCI while GitLab not). The student @chay2199 is more familiar with GitHub as well.

For the sake of this year's GSoC project, I propose:

1. Migrate https://gitlab.com/coala/coala-json from GitLab to GitHub. We can keep the GitLab repository for now, so that the student might want to switch between GitHub and GitLab when necessary.
2. Rename the repository to coala-reporter, or similar. The original name `coala-json` might be confused with `coala --json` mode, and the scope of this project seems to be sometimes out of the scope of 'json'.

ping @jayvdb

Start by reviewing the linked GSoC project and the existing coala-json repository on GitLab, then check whether a corresponding GitHub repository already exists. Confirm the migration destination and final repository name with the project stakeholders. Done means the repository is available on GitHub under the agreed name while the GitLab copy remains available as requested.

Written by the indexing model from the issue text.