left-sidebar: Update the border-radius of org icon.
left-sidebar: Update the border-radius of org icon.

Org icon now have rounded squares.

Fixes: #574.

zulip/zulip-desktop,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-electron