css: Fix hover color for "Edit topic" icon in recipient bars.
css: Fix hover color for "Edit topic" icon in recipient bars.

Previously, this had different hover behavior from the adjacent
elements, which seems like a bug.

The CSS for this component is shared with Recent Topics; we migrate
the styling for on_hover_topic_read for consistency.

Fixes #21273.

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