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
Hmm, wonder if you can check for the previous notification's priority. Alternatively, yeah, maybe not worth it.

I just wanted to be careful about overusing high priority notifications, because indirect messages aren't necessarily high priority, just normal priority. In 5.0, high-priority notifications get shown as heads-up notifications, so that's something to think about.

For now, I think I'll just remove it and see what the best option is later.