Addons: return all active versions on single version project (#11727)
* Addons: return all active versions on single version project
We always return all the active versions no matter the versioning scheme of the
project. That's how `versions.active` is expected to work.
However, it seems the conditional was below where these versions are calculated.
I also updated the test case that checks for the active versions on single
versions projects.
Closes https://github.com/readthedocs/sphinx_rtd_theme/issues/1613
* Add special case for single version projects