wp-fdroid: show GPG Signature link for source tarballs
wp-fdroid: show GPG Signature link for source tarballs

Now that source tarballs can be GPG-signed, the website should also display
a link to fetch them.

fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver