Losslessly compressed all images using optipng and advpng

app/assets/images/RSS.png 0.296 kb -> 0.295 kb
app/assets/images/about/osm.png 175.086 kb -> 173.764 kb
app/assets/images/about/sprite.png
app/assets/images/about/sprite@2x.png
app/assets/images/aol.png
app/assets/images/attribution_example.png
app/assets/images/browse/aerodrome.p.16.png
app/assets/images/browse/allotments.png