Move repo homepage files into own directory

Merged username-removed-124398 requested to merge NicoAlt/repomaker:124-repo-page-assets into master

This is how a empty repo looks like:

➜  repo git:(master)ls
assets  icons  icons-120  icons-160  icons-240  icons-320  icons-480  icons-640  index.html  qr_code.html  qrcode.png

➜  repo git:(master)ls assets
facebook.png  f-droid.png  material.min.js  page.css  roboto-fonts  twitter.png

This is how a repo with two apps looks like:

➜  repo git:(master)ls
assets          de.grobox.liberario_48.apk  icons      icons-160  icons-320  icons-640   index.jar     index-v1.json  qr_code.html
categories.txt  de.nico.ha_manager_26.apk   icons-120  icons-240  icons-480  index.html  index-v1.jar  index.xml      qrcode.png