feat(model): show desktop notifications for enabled streams

[myii]

• Linked to Notifications should match the settings in the webapp (e.g. stream-based notifications) #666 (probably only a partial fix so far)

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As discussed with @neiljp in this topic:

• https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Debug.20notifications

[neiljp]

@myii Thanks for the work on this 🎉 I've merged it manually into master with a few minor adjustments and some updated tests as the three commits ending in 034337a