Python 3 got rid of cmp() and related functionality, which broke two

sorting-related bits of code: One for sorting image srcsets, and one for

sorting entries in LocalDataQuerySet.
The image srcsets was easily converted to using sort keys, which is

preferred in Python 3 and is a bit faster than having a sorting

function. LocalDataQuerySet needs a more advanced comparison function,

so is now using functools.cmp_to_key(), which enables usage of our

sorting function by implementing a key-like wrapper that calls into it

for value calculation.

Unit tests pass on Python 2.7. We're not in position to test on Python 3

yet, though.