cla-bot should support renaming GitHub usernames

[BPScott]

Github usernames can change, cla-bot should store and deal with Github user IDs as an immutable token that relates to a GitHub account, rather than using the username.

Consider the following situation:

• @bpscott signs a CLA and gets their name added to a cla list
• @bpscott makes some contributions and they get signed off
• User @bpscott (Id 227292) changes his user name @bpscott-zzz.
• The same user now fails CLA checks
• A new account can sign up as @bpscott make contributions and they get signed off despite this user having never signed a CLA