Fix #4078 by refactoring 404.html

I moved 404 to the assets to avoid loading font and images from joindiaspora.

 #- Can I move the design to a 404-style.css file which will be in the public repository too ?
 #- This pull request proposes to insert the two font files and the cat image, do you agree ? Depend of amazon seems really bad for me.
Merge pull request #4079 from Flaburgan/patch-3

Fix #4078 by refactoring 404.html