Store some client settings

[gnprice]

There are a handful of user settings that are specific to the client, not to the account, and so we'll want to store locally on the device (following up on #13).

The local settings we have in the zulip-mobile RN app are:

• dark theme vs. light theme
• Open links with in-app browser
• Mark messages as read on scroll
• Language

We don't currently have the features that any of those would control:

• ui: Implement dark theme #95
• msglist: Make links touchable, opening in browser #71
• Mark messages as read on scroll, configurably #81
• Set basic i18n framework #275

No need to implement this before we have those; all of them have reasonable defaults we can use, certainly reasonable enough for an alpha. But once we have one or more of those features, then we can add the settings storage and UI for all of them.