tippy: Fix `cancel button` tooltip in compose box.
tippy: Fix `cancel button` tooltip in compose box.

Add `data-tippy-allowHTML="true"` in button tag and fix the tooltip
for cancel button in compose box.

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