updates mailmap entries (facebook#19824)

Author: sethwebster

.mailmap

@@ -147,6 +147,7 @@ Steven Luscher <react@steveluscher.com> <github@steveluscher.com>
 Steven Luscher <react@steveluscher.com> <steveluscher@fb.com>
 Steven Luscher <react@steveluscher.com> <steveluscher@instagram.com>
 Steven Luscher <react@steveluscher.com> <steveluscher@users.noreply.github.com>
+Seth Webster <sethwebster@gmail.com> <sethwebster@fb.com>
 Stoyan Stefanov <ssttoo@ymail.com>
 Tengfei Guo <terryr3rd@yeah.net> <tfguo369@gmail.com>
 Thomas Aylott <oblivious@subtlegradient.com> <aylott@fb.com>