profile: Display email

[sirpengi]

In the profile screen (#195 ) we need to show the user's email address if it is accessible, but not the fake email that Zulip creates for the api.

Also need to handle the user update event for null values of delivery_email appropriately.

Helpful links:

• Search for "delivery_email:" in https://zulip.com/api/get-events#realm_user-update
• getDisplayEmailForUser in zulip-mobile: https://github.com/zulip/zulip-mobile/blob/8133bf2b8/src/users/userSelectors.js#L318