Add support for new "send tab" push message

[rfk]

As part of the new send-tab system, we want to introduce a new device notification type that is explicitly for sending tabs. It would replace the current use of the more general "sync:collection_changed" message with one that's specifically about sending a tab.

The process for sending this message will be a little more involved than the existing ones, because we'll need to write the message payload into the new "pushbox" service. The procedure will look something like this:

• Sending device posts a send-tab message to /v1/devices/notify, including an encrypted pushbox payload bundle and the id of the target device
• We post the encrypted payload bundle to pushbox, and receive back an opaque message identifier
• We send the "send-tab" push notification to the target device, including the message identifier

(This might sound like we're doing a bunch of custom dev work on order to support the very specific use-case of send-tab; I'm hopeful that the ability of pushbox to support larger and more durable payloads is something we can make use of more generically in one way or another).

[eoger]

May I be assigned? I started work on this.

[vladikoff]

@eoger Done!!

[rfk]

This landed.