priv/static/images: Add pleroma-tan

Source: https://shitposter.club/objects/9be7c3f4-5a29-45a9-ac40-301312578a02 pleroma-fox-tan-smol.png is pleroma-fox-tan.png but the width got divided by two in The GIMP. All the pleroma*.png got optimised with mat2 and optipng/pngcrush, depending on their best results. [ci skip]
pirms 5 gadiem · bae96206d0
--- a/priv/static/images/pleroma-fox-tan-smol.png
+++ b/priv/static/images/pleroma-fox-tan-smol.png
--- a/priv/static/images/pleroma-fox-tan.png
+++ b/priv/static/images/pleroma-fox-tan.png
--- a/priv/static/images/pleroma-tan.png
+++ b/priv/static/images/pleroma-tan.png