imports: always import the whole kallithea module to use top level kallithea variables

This is slightly more lazy and might avoid some depeendency issues.

contributors: update

Also: welcome in 2020!

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.

First step in two-part process to rename directories to kallithea.
This first step is to change all references in the files where they refer
to the old directory name.