CSS tweaks

This tweaks the stylesheet a bit:
* (HTML) We don't need the `id`s `requirements-link`, `installation-link`, so removing them (and some JS in order to not get uncommented 'cause it won't work any more without the id)
* (CSS) Unify `#requirements-link, #installation-link, #other-downloads` to just one rule
* (CSS) Make the line including these links wider to avoid wrapping
* (HTML) [contribute.html] Make `Starter Issues` a link

@larz0