All C extensions are now treated on equal footing and can be configured
in "build.conf".  All the options known by Extension may be set now for
any extension individually and for all of them together (using the
[DEFAULT] section).

Summary of changes:
* Add a test for trailing whitespace, remove all trailling whitespace
* Add a test for broken URLs in docs, fix all broken URLs and permanent redirects
* Switch sphinx doc build to strict (warnings to errors) and nitpicky (broken links to warnings). Fix all sphinx warnings.
* Deploy docs from any commit to [kwant/kwant](https://gitlab.kwant-project.org/kwant/kwant) to https://test.kwant-project.org/doc/dev
* Deploy docs from any commit to master at kwant/kwant to https://kwant-project.org/doc/dev
* Deploy docs from any tag release tag (so anything that is vX.Y.Z) at kwant/kwant to https://kwant-project.org/doc/X.Y

I try to cleanly separate between information that should be part of the
Kwant source distribution (that is tied to a particular version of
Kwant), and generic information that is best presented on the website.