Support streams:public in search, and offer it when search finds no results

[gnprice]

When a new user joins a realm with lots of history, Zulip search by default won't include any of the history they have access to from before they joined. This can be frustrating when trying to find information from previous discussion.

In web, we make this a reasonable UX by (a) having a streams:public operator that includes all history in public streams, and (b) showing a bit of help text when you try a search with no results, with a link that adds that operator to your query. We should do something similar in mobile.

Chat thread: https://chat.zulip.org/#narrow/stream/48-mobile/topic/search.20public.20streams.20fallback/near/1573488

Related issues:

• Search syntax in general: Support search syntax #4903
• A different way of making search syntax more readily usable, by offering it as typeahead in the search box: Search syntax suggestions #4916