luci-theme-openwrt-2020: link hostname to index page

In the Material theme, clicking on the logo takes you to the status page.
This seems logical, and is very helpful. But in OpenWrt 2020, clicking on
the hostname doesn't do anything, so just pulling the same href from Material
so the link works in this theme as well.

Signed-off-by: Russell Morris <rmorris@rkmorris.us>
[adjust subject, commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>