compose: Make topic input uncontrolled.
compose: Make topic input uncontrolled.

Remove the `value={topic}` from topic text input, making it
an uncontrolled component.

To replicate the previous behavior:
 * initialize at the start in `componentDidMount`
 * make sure the topic input text is updated when we focus on
   the message input (it is invisible until then)
 * change the text on picking a topic autocomplete suggestion

Fixes #2589.

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