Make fastText an optional dependency

[osma]

The fastText package needs the native library to be compiled during install. While it has worked reasonably well, I think it would make sense to make all non-pure-Python dependencies optional so that the core Annif can be installed without any compilation of native code.

So the fasttext (actually fasttextmirror) dependency should be moved from install_requires to extras_requires in setup.py and similar for the Pipfile. The backend code should check whether the fasttext module can be imported and fail gracefully if not. Unit tests should also notice if fastText is missing and skip the tests that require it.