This converts the Indexer class from something that strictly generates
index.html files for folders to a more generic class that will be able
to hold other types of transformations we might like to do on the
total collections of uploaded files.
The index.html specific arguments are moved into make_indexes() and
the two helper functions that should not be called externally are
renamed private.
Change-Id: I388042ffb6a74c3200d92fb3a084369fcf2cf3a9
2d61e9d4fc