Revert "remove submodule otp.js"

This reverts commit 55aa7b25af4c1dd1d10f746e448bbfd035f483a8.
Revert "Set up submodule for otp.js."

This reverts commit 2c83aba5055164ca5365c337d4410912eb0a72d3.
Check otp.js into the repo directly for now.