Also update CI to install qsymm on relevant platforms, and update
pytest config to skip kwant.qsymm tests when qsymm is not installed

The UI will show the test failures as warnings (which we can
address).

Sometimes the testing jobs fail because the build artifacts
are removed.

Only builds testing images when the relevant dockerfile/conda envs
have changed.

Currently we test against 5 environments: Ubuntu LTS, Debian stable,
minimum required dependency versions from conda forge, minimum
required dependencies without extras (sympy and matplotlib), and latest
dependency versions from conda forge.

So it's no longer necessary to provide the REFNAME environment variable.
Also the link template string format is now more general to support at
least gitlab github and cgit.

Gitlab now has built-in mirroring, so we don't need to have a CI
step for this now.

Gitlab now has built-in mirroring, so we don't need to have a CI
step for this now.

Closes #171

sympy is installed in the Kwant testing image now.

We build conda packages on 'master' (on all forks) by default.
Builds for branches may be triggered manually.

Conda forge removes support for Python 3.4. If anyone is using
conda then they can easily upgrade to newer Python in any case.

Previously we used 'ssh-keyscan' on every upload, so derived
no security from host key checking. 'ssh-keyscan' caused the CI
server's IP to be blacklisted, so now we just ignore the host key
entirely. We never send private information, so this is fine.

We previously restricted conda package uploading to master branch on the
main Kwant repository, as otherwise this step would fail for forked
repositories without a valid anaconda key. Now we build the conda
packages and upload to anaconda in two separate jobs, and the upload job
is only run on the main Kwant repository.

Without this measure, a failed check is no longer detected.  This seems
to be a bug in gitlab CI.

The CYTHON_TRACE macro can be now set in the build.conf file.

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