Homebrew and Python

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/11361

This PR does not make an explicit choice between various flavors of Python but rather 
- describes both options i.e. system-wide vs Homebrew Python and highlights their specificities during the OMERO installation. 
- adds links to the Homebrew and Python Wiki page describing the Python formula and its pros/cons
- adds a couple of instructions about virtual environments as well as a link to the Homebrew installation script.
- fixes the environment variables block (remove deprecated `BREW_DIR` and remove `$ICE_HOME/python` from `DYLD_LIBRARY_PATH`)

/cc @bpindelski