Bug 894055: Add funnelcake parameter to bedrock URLs.

Now if a page has a download button on it, and the
url includes the query param `f=[0..9999]`, then that
number will end up on download links in the form
`product=firefox-stub-f999`. The ID must be numeric
and not exceed 4 digits.
Merge pull request #1092 from pmclanahan/enable-funnelcake-qs-param-894055

Bug 894055: Add funnelcake parameter to bedrock URLs.