right_sidebar: Highlight search icon when hovering userlist_header.
right_sidebar: Highlight search icon when hovering userlist_header.

Clicking the entire userlist_header opens the user filter, however,
previously only hovering over the user_filter_icon would cause a
highlight effect. This commit changes the behaviour so that hovering
over the userlist_header would also cause the same highlight effect on
user_filter_icon.

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