replace fmt_source_code_url in strings.signed_by_developer too
As reported on matrix, currently there are two source tarball links for apps like NewPipe using both upstream's and our signature, the second one being broken:
https://f-droid.org/repo/org.schabi.newpipe_994_src.tar.gz
https://f-droid.org/packages/org.schabi.newpipe/{{ source_code_url }}
I think this change should fix it but I haven't been able to test it.
I also noticed that a lot of translations don't include the <a href=\"{{ source_code_url }}\">this source tarball</a>
bit, so there is no link at all for those.