Make the distinction between source and platform releases clearer

The current wording seems to make the difference too subtle for a casual
reader; try to clear this up by:

- naming the latest release "source" instead of "stable" release, to
  make clearer what it is that we're showing the version for.
- reword the button to "Downloads" for a platform, which makes it a noun
  for the available downloads, rather than a verb which can too easily
  be taken to mean "download this version"

This should reduce the incidence of people complaining that the version
in the front is erroneous.

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Ideas from #268. Possibly not perfect, but it should help.