Correct licensing information in individual files. The top-level license file is now LICENSE.md. Also, in various places where there should have been joint copyright holders listed, a single copyright holder was listed. It does not appear easy to add a link to a large list of copyright holders in these places, so it simply refers to the fact that various authors hold copyright. In future, if an easy method is discovered to link to a list from those places, we should do so. Finally, text is added to LICENSE.md to point to where the full list of copyright holders is, and that Kallithea as a whole is GPLv3'd.