Separate unstable download information to a separate file, make
inclusion conditional on $latest_unstable.

The idea being that if $latest_unstable is set to False in
externals/global_variables, the unstable section will not display.
First testing to ensure that the include bit works.

git-svn-id: svn+ssh://svn.gnucash.org/repo/htdocs/branches/beta@22690 57a11ea4-9604-0410-9ed3-97b8803252fd