icons: Set font-display: block for icon font.
icons: Set font-display: block for icon font.

It’s not helpful for the browser to substitute another font for the
icon font while it’s loading.

This suppresses a warning from the Lighthouse performance analyzer.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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