compose: Match width of topic compose to left sidebar space.
compose: Match width of topic compose to left sidebar space.

Previously, the maximum width for the topic input in the compose box
was artificially limited to 20% of the width of the compose box.

While this may have had some useful role in encouraging short topics,
we can teach that idea in other ways, and it seems more helpful to
have the input length match what works well for viewing topics in the
left sidebar without being cut off.

andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,kou/zulip,andersk/zulip,kou/zulip,andersk/zulip,andersk/zulip,kou/zulip,rht/zulip