search: Handle all ways of searching (`sender:`, `is:unread`, etc.)

[chrisbobbe]

Our initial implementation of search won't be more powerful than zulip-mobile:

• msglist: Offer search narrows #252

This issue is to support the full range of Zulip search features. It calls for more complicated UI than a plain text field.

Related issues

• Parse all valid narrow links, opening search screen for complex ones #1661

[macieksarnowicz]

Having the full power of the desktop app search in mobile would be fantastic - for example, searching within a channel or topic.

[gnprice]

When we add operators like channel: and dm: that correspond to existing narrows in the app, we can also extend #1854 by adding a search button to the app bar in those narrows. (It would lead to a search screen with the corresponding filters already filled in, so as to search within the previous narrow.)