Qsymm is now installed in the testing image directly

This reverts commit f2ac01b9.

We still have to manually start image building jobs, and there is
no disadvantage to allowing rebuilding arbitrary images.

Latest versions of Kaniko builder use this new credentials directory

Use a more modern sphinx invocation

The build process uses a recipe from a separate repository, which was not
updated for a while.  Instead we should be calling conda-build directly.
However this may be done separately.

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.