gopherbot: change congratulateNewContributors to use email as identifier
gopherbot: change congratulateNewContributors to use email as identifier

Instead of using the full author string, "Foo Bar <foo@bar.com>", just use the email to avoid congratulating users when they use a different name.

Fixes golang/go#21731

Change-Id: Ib1ce4de76eeb60de91f7af705946a630d943659e
Reviewed-on: https://go-review.googlesource.com/61030
Reviewed-by: Kevin Burke <685caccd7011cfe58b8f9ef79ff9dca388f03c79@inburke.com>

golang/build,golang/build,golang/build,golang/build