Introduce per-repo default reviewers

[tnull]

We allow the Github bot to assign a per-repo default reviewer for the first review.

cc @TheBlueMatt

As usual didn't have a chance to test this locally, let me know if I messed up somewhere.

[tnull]

Force-pushed with the following changes:

> git diff-tree -U2 6ef3159 0051a82
diff --git a/github_bot.py b/github_bot.py
index 38fb786..543886a 100644
--- a/github_bot.py
+++ b/github_bot.py
@@ -468,5 +468,5 @@ class GitHubBot:
                # reviewer for the given repo, assign them and return early.
                if not current_reviewers:
-                       default_reviewer = DEFAULT_FIRST_REVIEWER[repo_name]
+                       default_reviewer = DEFAULT_FIRST_REVIEWER.get(repo_name)
                        if default_reviewer is not None and default_reviewer != pr_author:
                                self.assign_reviewer(repo_name, pr_number, default_reviewer)