Track realm settings

[gnprice]

Zulip has a long list of realm settings. Many of them are relevant to the mobile app. This issue is for beginning to track a few of them; once we do that, it'll be easy to add individual settings when they come up.

The settings are documented as part of the main event that updates them:
https://zulip.com/api/get-events#realm-update_dict
Note there's a different event that can also update them:
https://zulip.com/api/get-events#realm-update
and so we need to support both.

The settings are also present on the initial snapshot, generally with a name realm_foo for a realm setting named foo.

For a model for how this can work, see our handling of user settings, in particular UserSettingName. The goal is to set up a system where adding any one new setting involves adding only a small number of lines of code in a small number of places. The PR for this issue should end with a commit that demonstrates that, by adding one more setting, just like #261 did for user settings with its last commit 4bcbffd52.

[gnprice]

Pushing this back to the post-launch milestone — we can pull it forward again when we identify a specific feature that depends on it.