Test GitHub API limits

[thereseanders]

The class size that can be accommodated for the peer review process via ghclass is capped by limits on calls to the API.

We need testing for

• How many students the peer_* functions can accommodate for different numbers of reviewers?
• What are the limits on the data size (e.g. for final projects) that can be transferred via the GitHub API for a large number of students?

[rundel]

I was able to get this PR merged into gh during tidydevday - this gives us easier access to the api rate limit information which ghclass can hopefully act on or at the very least report back to the enduser.

Installing the dev version from github should get you the gh_rate_limit function, not sure when Gabor plans to push things out to CRAN.