Serve apps from multiple repositories

Like the client the plugin should be able to serve apps from multiple repositories.

If this is added, https://gitlab.com/fdroid/fdroid-website/issues/57 can be done.

mentioned in issue fdroid-website#57

mentioned in issue #30

mentioned in merge request fdroid-website!92 (merged)

For the use case of making a custom webview of multiple repositories, we have Repomaker. So we shouldn't duplicate that here in the jekyll plugin. Instead I think we should move towards a strict definition of a repo as having a repo and an archive section. repo is required and archive is optional.

Then on the website, I think we should show the archive somehow, but I don't think it makes sense to just merge them into one app listing. The website should mirror the repository and fdroidclient as much as possible. To mirror that UX exactly, we'd need to have a per-user "enable archive" preference in the website. At first thought, that doesn't sound very feasible, but maybe.

So perhaps we want to have a separate listing for the archive.

mentioned in issue #45