Add notify type preference.

WIP: still testing to make sure this works as expected

![image](https://cloud.githubusercontent.com/assets/4513759/4461747/f6cedbd2-48bc-11e4-8a20-39533c57a5e6.png)

This adds a preference to not receive all messages, and only mentions/1-1 messages (like the official app) or maybe just 1-1 messages.

Depends on https://github.com/Neamar/notiflow-server/pull/7
I had a look to see if it was possible to get a notification, and it doesn't look like `NotificationManager` provides that, oddly enough. We could store the priority of the notification in `NotificationHelper` and check if it's already high to avoid lowering it?

There is merit to your concern, so I don't want to be quick to dismiss it. I'm wondering how this would work out... assuming the user is getting notifications for all messages - they get mentioned in a flow, so that raises a high-priority notification. Subsequent messages are probably related, so you'd want to keep the incoming notifications as high priority, so you can easily follow along (especially with heads up notifications).

However, then again, there might be an unrelated discussion in a different thread at the same time which isn't high priority. So now we would need to check which thread the message is in (which is the odd `influx` tag in the message, by the way). Although, since notifications are per-flow, we would accidentally lower-priority on the notification because it was in a different tread (even though it contains a high-priority message), but we would incorrectly treat the new message as high priority if we didn't.

On the other hand, you could look at high priority as more of an immediate effect, so as long as the notification was high priority, that's all that matters because the user is now aware. I think the only effect that lowering priority has right now is it gets put lower in the list of notifications.

tl;dr - options are
- Treat all messages as high priority (undo change)
- Treat only the first message as high priority, lower the message back down to normal priority for subsequent messages (keep change as-is)
- Keep track of last priority in order to be able to keep the message as high priority