fix bug introduced in 31667478: when the page was displayed on a phone the download icons were not displaying correctly

In particular:
— always use double bracket operator
— remove unneeded quotes around constant strings, when assigning variables or
  within the double bracket operator
— use consistent indentation style for compound constructs

— factorize the two scripts into a single one
— various safety and style improvements

– no longer use a dedicated runner
– use the bundler from the Debian package
— store the gems in a per-project/per-branch cache, instead of globally for all
  runner users

Otherwise I can’t manage to make recursive cloning work on my machine.