coreos-download: use releases instead of builds for release note
    
    Fixes the issue that releases are subset of builds and the displayed
    information should be between releases instead of builds. For example,
    if there are three builds between two releases, the pkgdiff between releases
    should be accumulated among all three builds in between plus the pkgdiff
    between last build and the current release.
    
    Also cleaned up naming inconsistencies in coreos-download.js, use camelCase
    naming convention if possible. Also adds comments on functions and hides
    buttons for links to fcos release browser for security and consistency concerns,
    since the release browser is subject to change without notice.
    
    Signed-off-by: Allen Bai <abai@redhat.com>