Revert to old-style data formats

Here's my attempt at  #97, removing the new-style JWT-compatible formats and reverting to the old-style format as the only option.

A rough list of the changes made to accomplish this:

  * Revert "kty" field to "algorithm"
  * Revert "pubkey" field to "public-key"
  * Revert "sub" field to "principal"
  * Revert "DSA" to "DS" and "RSA" to "RS", removing notion of algorithm name aliases
  * Revert all timestamps to milliseconds rather than seconds
  * Revert b64-encoded field values to previous decimal or hex encoding
  * Remove "newFormat" and "version" switches in code and docs

@callahad what have I missed? :-)

I will test this with https://github.com/mozilla/fxa-auth-server/pull/831 and a full fxa+sync stack deploy and report back.