User is collaborator check

[davidalber]

GitHub (now) has an endpoint to check if a user is a collaborator. Currently, Highfive uses the list collaborators endpoint and then checks the response to see if the expected user is there. I assume this response is subject to being paginated in projects with lots of collaborators. Highfive, however, isn't handling pagination in this case. I don't know if that's an issue (i.e., I don't know how many contributors rust-lang/rust has; a problem would manifest as Highfive ignoring "r? @" comments from some collaborators), but it seems like an issue waiting to happen.

This PR modifies Highfive to check if a specific user is a reviewer.

In addition to the unit tests, I manually verified that is_collaborator is working as expected. Here's the log of that:

>>> from newpr import is_collaborator
>>> is_collaborator('blah', 'davidalber', 'highfive-test-repo', 'davidalber', OAUTH_TOKEN)
False

>>> is_collaborator('davidalber', 'davidalber', 'highfive-test-repo', 'davidalber', OAUTH_TOKEN)
True

>>> is_collaborator('nrc', 'davidalber', 'highfive-test-repo', 'davidalber', OAUTH_TOKEN)
False

Here is some justification for caution: I haven't tested this on a Highfive instance running against a repository. (I'm working on this now, but I keep accidentally opening my PR here. Rather than closing this again and making more noise, I am just going to update this comment when I'm done.)

[davidalber]

Sorry about #110. GitHub tricked me (er...I wasn't paying enough attention) twice to open a PR in this repo, when I meant to put it in my fork of this repo.

Anyway, I tested the new_comment code path that calls is_collaborator in davidalber#2. Looks good!

Thus, with the unit tests and the manual end-to-end test, this appears to be working correctly to me.

[nrc]

This seems like a better approach, thanks!