www/adguardhome: Use pre-bilt html frontend tarballs and mantenance mode which allows to build them.

Port was broken without this since they recently added the need for html
frontend being presend before build and such frontend was missing.

Reported by: Sandro Wirth <sandro.wirth@posteo.de>

(cherry picked from commit 990e47d4e491bbed9bdd626b4b89f186de5804c7)