…/v2 (#522)

In an older version of the GitHub API, getting the contents of a large
file returned a 422 response. This is no longer the case. Instead, you
get back a standard JSON response with an empty content field. Simplify
the logic for loading config by making a single call to the contents
API, then using the response content to determine if we need to use the
direct download URL or not. This also fixes a long-standing bug with the
appconfig package when loading files larger than 1MB.

I verified the API behavior against GitHub Enterprise 3.17, so I believe
it will be the same on GitHub.com (this also matches the API docs.)

I discovered this as part of trying to upgrade to go-github v85, which
refactored the DownloadContents function in a way that broke our tests.

[skip ci]