#437 Allow easy switching between website versions

• Best to review per commit.
• Changes:
  • Adds version switcher to the top-right of the 12 fixed website pages. Separated the version number and release notes links to make this possible. Only some versions are shown in the dropdown menu, to prevent a very long list.
  • The list of versions is obtained through JavaScript, from a .versions file on the website. This file does not exist yet. It will be added once this merge request is approved, and before it is merged. After that, the file will be automatically updated when a new website version is added or removed.
  • Improved styling of dropdown menus, also for existing ones.
  • Added a versions.html fixed website page that lists all website versions.
  • Added link from download.html to versions.html.
• Testing this is somewhat difficult. I'll explain that in a separate comment, with examples of what it looks like now.

Addresses #437 (closed)