Lossless optimization of PNG images

Optimize images using pngstripper, optipng and advpng (using these settings).

Here are the new smaller sizes compared with the old ones:

-rw-r--r-- 1 francois francois 60735  2 oct 13:29 public-new/static/card.png
-rw-r--r-- 1 francois francois 87472  2 oct 13:38 public/static/card.png
-rw-r--r-- 1 francois francois 13466  2 oct 13:29 public-new/static/fdroid.png
-rw-r--r-- 1 francois francois 22702  2 oct 13:38 public/static/fdroid.png
-rw-r--r-- 1 francois francois 10355  2 oct 13:29 public-new/static/gajim-logo.png
-rw-r--r-- 1 francois francois 11110  2 oct 13:38 public/static/gajim-logo.png
-rw-r--r-- 1 francois francois  9180  2 oct 13:30 public-new/static/google_play.png
-rw-r--r-- 1 francois francois 19687  2 oct 13:38 public/static/google_play.png
-rw-r--r-- 1 francois francois  8819  2 oct 13:29 public-new/static/jmp_beta.png
-rw-r--r-- 1 francois francois 12282  2 oct 13:38 public/static/jmp_beta.png
-rw-r--r-- 1 francois francois  6627  2 oct 13:29 public-new/static/pay_with_bitcoin-lukasz_adam.png
-rw-r--r-- 1 francois francois 10276  2 oct 13:38 public/static/pay_with_bitcoin-lukasz_adam.png