Add an option to order positions by the person's legal_name

This is part of the fix for issue #422.

The positions.distinct('place') in finding child places seemed
to be forcing an ORDER BY on the place name, although I would
have thought that calling the distinct method just returned a
new QuerySet rather than affecting positions as a side-effect.
In any case, that call is easy to remove, since we were using
set on the results anyway.
There's no need to explicitly sort the returned places by name

The Place model already has a default ordering on the place's
slug, which will sort the places sensible anyway.
Kenya: make menus link to MPs / Senators sorted by person name

Fixes #422
Preserve the order when switching between grid and list position views
Update a now-outdated comment
New favicon for Nigeria. Closes #454
Use http://www.mysociety.org instead of its robots.txt in a test

Since the site redesign there has been no
robots.txt file for mysociety.org, but one of the hansard
tests assumed it would be present, as an arbitrary file
to fetch and test caching of.  Any web page will do, so
just use http://www.mysociety.org

Fixes #680
Remove uses of original_id from test fixtures

Fixes #679
Merge branch 'name-sorting'
Incorporate position-sorting changes into production