Group ids should be strings (or stringables)

ArrayBuffer was a bad choice.
Don't validate presence of thread id

It's undefined until the first save();
Make initial timestamp on a thread match the 1st message
Use textsecure.messaging to create groups

Not textsecure.storage. Sigh. Also accomodate the fact that
the group id is not returned directly, but rather at the end
of a promise chain.
Add phone number utils to background page

Quickfix.
Probably it shouldn't actually be verifying any phone numbers.