Improve logo on splash page for high-res screens

Summary:

• The PNG variant is a rendering for 1dppx, which looks blurry on high-DPI screens.
• The SVG variant is smaller than a 1.5x or 2x PNG would be, and only marginally larger than the 1x PNG (10K vs 11K), so simply use the SVG always.
• Where the PNG was cut to the edge, the SVG has some in-canvas spacing around the graphic. In order to keep rendering the same, change the image from 'mr-3' to 'mx-1', which effectively changes 'margin-right: 1rem' to 'margin-{left,right}: .25rem'.

Reviewers: bd808

Reviewed By: bd808

Differential Revision: https://phabricator.wikimedia.org/D1018