Add the unmailmapped committer email from the GetCommitSignatures response to signatures

What

Add the unmailmapped committer email from the GetCommitSignatures response to the signatures object in Gitlab rails.

Why

We need to use the unmailmapped committer email from the GetCommitSignatures response to determine if the commit committer email has changed from when the commit was verified.

This wont be possible until until work on GetCommitSignatures RPC returning unmailmapped committer emails is complete. This should be very similar author emails from the GetCommitSignatures RPC were handled: !171258 (merged)