[cmake] Use local MathJax version if available

This will search for a local version of MathJax and use it if found. If not, we fall back to the cdn version. This should e.g. find a MathJax version installed as Debian package.

The behaviour can be configured using the two CMake variables DUNE_FUNCTIONS_MATHJAX_DISABLE_LOCAL and DUNE_FUNCTIONS_MATHJAX_DISABLE_CDN which allow to disable the local and cdn version independently. The former can be used to disable the local version e.g. to get a portable doxygen documentation (with or without cdn MathJax). The latter allows to disable the cdn version to safely avoid a 'privacy breach' even if there's no local MathJax.

A few comments on the implementation:

• This requires to provide a patched version of dune-common's prepare_doxyfile macro in order to pass the MathJax options to Doxygen.
• One may want to add the patched macro to dune-common.
• The check for MathJax could be extracted as FindMathJax.
• The handling of the two user variables could also be moved to dune-common, since the usage in a module is opt-in anyway.
• Someone with more CMake experience should check the code.