Tattoo enhance

Tattoo app seems to be broken in owncloud 5.

This fix the app + add some enhancement by using img instead of css for displaying background.
With css, the background image is "cut" by other div (for example, you may not see the background image in the files list if there is too many file rows).
I use an img, with position absolute, bottom right like the css background, z-index and small opacity to prevent visibility problems.

See #857 and #858 for previous (and abandonned for making a clean one) pull-requests.