Issue 6765 - Added COPYING file

This MR is for #117 (closed).

While trac#6765 is about including the already existing COPYING file in the extension bundle we ship, this change in particular is about adding a COPYING file to the repository that contains the UI code.

@Zarillo I merely copied the COPYING file that we've been using the in the web extension repository to the UI repository so that we also have it there. Please let me know if there's anything else we need to consider in that regard.