Prevent rasterization artefacts in the logo, and simplify markup.

Prevent rasterization artefacts in the logo, and simplify markup.

• Remove sketch-namespaced attributes.
• Remove unused groups, and cancel out unnecessary transforms.
• Use overlap to avoid rasterization artefacts in the logo.
• Extend comment for #logo click event handler, as it's a non-obvious hack for Safari.
• Remove viewBox parameter and define paths in their final form.