support adding arbitrary files to a repo

This adds the most basic level of support for including arbitrary files in
an F-Droid repository.  This is useful for things like including videos,
ebooks, update.zip files for ROM updates, and more.  The aim is to have
this as generic as possible to keep it flexible for unforeseen uses.

Code-wise, this is really just a first effort.  This area of code has not
been touched in a very long time, and the repo parsing is done in a giant
function that is not easy to break apart.  It should be broken up to more
cleanly support arbitrary files.

Also remove the TODO line, we've decided to keep the old permission
format for now, at least until there is a major overhaul of the index
data format.  And the issue tracker the proper place for TODOs.

fdroidserver/common.py

@@ -1609,6 +1609,12 @@ class KnownApks:
         return lst
 
 
+def get_file_extension(filename):
+    """get the normalized file extension, can be blank string but never None"""
+
+    return os.path.splitext(filename)[1].lower()[1:]
+
+
 def isApkDebuggable(apkfile, config):
     """Returns True if the given apk file is debuggable