Add Fira font files to project

Because the GitLab CI can’t run apt-get at the moment, the font must be
embedded into the project instead of installed in the container.
Also, a switch is added so that if a machine has the font installed, it
uses that version. This is needed because LuaLaTeX doesn’t like using
the project .ttf files on my machine. So this is a workaround for that.