MDL endpoint no longer resolves in MDL.less, breaking site functionality.

MDL endpoint in line 1 of /src/style/mdl.less no longer resolves, as MDL has reached end of life. Can be temporarily fixed by replacing with an active endpoint that still resolves (like //cdnjs.cloudflare.com/ajax/libs/material-design-lite/1.3.0/material.deep_purple-pink.min.css). Long term, should probably transition to Material Design 3 for Web. 