wizard-downloads-page, search: Don't duplicate recommended downloads
The recommended downloads are OSes defined in the recommended-downloads.xml file. Some of them are already retrieved from os_db.list_downloadable_oses, so there's no need to duplicate them.
Custom OSes, such as the Free RHEL option, still need to be added. A custom OS is an OS Media that doesn't have a defined URL, since it will be handled specially (a login gate).
Fixes #374 (closed)