Add "Available for Fedora" section to the explore page

Two motivations for this change:

1. Historically, I've been of the opinion that we should advertise big name apps in Software when they are available, even if they are proprietary. While the free software purists baulk at this, for the vast majority of users, availability and easy installation of recognisable apps is a very positive thing.
2. In Fedora there's a 3rd party repos feature that allows users to enable a set of additional repos, to make software available that isn't included in the main repos, including some proprietary software. Since we have this feature to make more software available, we need a way to make it discoverable and visible to users.

The proposal is to include an additional "Available for Fedora" section in the explore page, similar to the current editor's picks. This would include a set of apps which are specified downstream, and would include apps that are available through the 3rd party repos feature.

Depending on the number of apps in the "Available for Fedora" section, the selection that's presented could rotate over time.