Add link to source code repository

### Problem

It would be nice to have a link from the AppCenter page to the project's source code repository. There currently is no way to locate the source code repository of a project from its AppCenter page.

### Proposal

For example, in https://appcenter.elementary.io/io.elementary.calculator there could be a link to https://github.com/elementary/calculator, perhaps next to the "Download as Flatpak" button at the bottom. Maybe even replacing the "Publish on AppCenter" link, which IMO should be in a site-wide location like the header or the footer, rather than in a location that makes it seem like it's related to the particular package being viewed. Like this:


If there's agreement on this proposal, I'd be happy to contribute a PR implementing the first step, i.e. moving the "Publish on AppCenter" link to the site footer.

### Prior Art (Optional)

_No response_