compose: Make input value updates more consistent.
compose: Make input value updates more consistent.

A couple of spots in `componentWillReceiveProps` were calling
`setState` directly to update `message` and `topic`.  Instead,
use `handleMessageChange` and `handleTopicChange`, which do those
and also take care of related logic.

This ensures consistency with other functions in the same component.

This changes the behavior slightly:
 * for topic, we collapse the compose buttons;
 * for message, we do the same, we send a 'typing' event
   to the server.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile