Use an SVG logo by default. Fallback to PNG if SVG is not supported

Signed-off-by: Pooria Azimi <pooriaazimi@me.com>
refactor fallback method

Signed-off-by: Pooria Azimi <pooriaazimi@me.com>
use relative path for logo.svg - otherwise, phpBB/adm/index.php fails to display it

Signed-off-by: Pooria Azimi <pooriaazimi@me.com>