split sms & email verification 3rdparty code

We might want to bundle the code in `3rdparty`.

React & presentational components don't belong in there. At the
same time, the terms of service are strictly related to the use
of these external services. We decided to not bundle them, but
still keep them in a file called `terms-of-service.js`.

The commit also moves the "how it works" section into the
presentational part in `modals/Verification`.