coreos-download: revert main.py and use url param
Reverts https://pagure.io/fedora-web/websites/pull-request/99#
since using routing to different tabs introduces more overhead
and over-complicates the problem.
For the need of querying different tabs under /coreos/download,
a query parameter `tab` is used and set when clicking on tabs.
This is much simpler than adding routing and redirect to tabs
with routes.
Usage:
- http://localhost:5000/en/coreos/download/?tab=cloud_launchable
- http://localhost:5000/en/coreos/download?tab=metal_virtualized
- http://localhost:5000/en/coreos/download/?tab=cloud_operators
Closes: https://github.com/coreos/fedora-coreos-tracker/issues/417
Signed-off-by: Allen Bai <abai@redhat.com>