Update versions for current release
Update available versions for each channel; amend download table

The release notes row no longer makes any sense at the bottom
of the download table, as the major version for Android beta
is different from the major version for Tizen beta. Moved it
into a bulleted list instead.
Remove download links from channels viewer

The download links don't point at valid downloads any more;
and the version available for download doesn't correspond to the
channel/branch anyway (e.g. the branch shown for Tizen beta is 5
but we don't have any Tizen beta 5 downloads).