How to get closed issues?

From: https://developer.github.com/v3/issues/#list-issues-for-a-repository
To get the list of issues of a repository we need to send a GET request to ```/repos/:owner/:repo/issues```

By adding the ```state``` query parameter to the query we can choose to get all the issues and not just the open ones:

We already have the ```get_issues``` function: https://github.com/dwyl/github-backup/blob/295ee9e7cf0bc9b5f6d88e89601c5341a2351956/lib/app_web/controllers/github_api/http_client.ex#L47-L52
We need to update this function to add the ```state``` query parameter with the value `all`

ref:
- https://github.com/dwyl/github-backup/issues/9#issuecomment-368071789
- third checkbox of https://github.com/dwyl/github-backup/issues/24