[#320] Resolve "restore standalone number registration flow"

Closes #320 (closed)

context

• we used to have a standalone boost register command for registering already purchased twilio phone numbers with signal
• we got rid of it because in almost all cases we would register in the same step as we purchased (via provisionN)
• however, in some cases (such as retrying after a failed registration or when trying to re-register a number in order to blast away a borked keystore) it would be useful to have a standalone register flow

changes

• restore the phoneNumbers/register endpoint
• restore the boost register command that hits that endpoint
• get rid of some signal-cli doc cruft turned up by grepping for cli