Make some download links static

As the beta and stable channels do not automatically synchronise with the versions on the corresponding github branches, make the links for those channels in the download table static (i.e. via versions.js).

Similarly, the channels viewer links to specific tarballs/rpms which may not exist, so fix this to point at the download directory for the channel instead.

NB this also changes the link creation logic slightly, as tizen-mobile downloads switched from a "586" suffix to "686".

Fixes https://crosswalk-project.org/jira/browse/XWALK-1016