🐑 naive alt-store support

Naive shot at implementing alt store support. Might still be missing important
bits and pices I'm not aware of.

fdroidserver/common.py

@@ -4155,6 +4155,7 @@ def is_repo_file(filename, for_gpg_signing=False):
     if isinstance(filename, str):
         filename = filename.encode('utf-8', errors="surrogateescape")
     ignore_files = [
+        b'altstore-index.json',
         b'entry.jar',
         b'index-v1.jar',
         b'index.css',