update license link to LGPL 3
create download page for (version, distro) only when installers exist

Previously, created every page in (all-versions X all-distros).
handle new installer info lines
change all versions download page to accommodate racket-minimal distro

Change from fixed 2 options (Racket, Textual) to flexible
(Racket, alternatives).
add link to planet from community page
add symlink to releases dir