Add support for SVGs to $wgLogoHD with PNG fallback

Add support for SVGs to $wgLogoHD with PNG fallback

SVGs could already be used through $wgLogo. However, if a PNG fallback
is desired for older browsers, using SVGs was previously not possible.

This commit adds support for using an SVG image in $wgLogoHD and,
using $wgLogo as the fallback image.

Usage example:

> $wgLogo = '/path/to/png';
> $wgLogoHD = [
>     'svg' => 'path/to/svg',
> ];

Note: When the 'svg' key is set in $wgLogoHD, any '1.5x' and '2x' keys will
no longer be used because SVGs can render optimally on any screen sizes.

@Reedy, @Krinkle and @Brion VIBBER helped me alot with this.

Bug: T86229
Change-Id: I6197d96ce9110f4711ef2c4b198445bc5c6ae110