Set license files and headers for bitcoin.org's content

The license for the content on bitcoin.org needs more clarity.

Although the MIT license was mentioned in the footer of the website since the first commit, the website didn't provide licensing information in the usual locations (file headers or COPYING files), except for the libraries and files under different copyright owners or licenses than MIT.

This pull request fixes that issue while carefully avoiding ambiguity or conflicts between multiple licenses with a per-file or per-folder licensing. For this reason, all images under img/ have been moved to a subfolder (I should have carefully tested all layouts with this change).

In the absence of critical feedback, this pull request will be merged on November 28th.

A list of files and folders under a different, or slightly different license or copyright owner:

/_less/normalize.less
/csshover.htc
/font/ubuntu
/font/droidnaskh
/js/leaflet
/js/leaflet-markercluster
/img/flags
/img/brand
/img/wallets
/img/screenshots
/img/faq
/img/innovation
/img/press
/img/os