Add an AlternativePersonName model to replace Person.other_names

The motivation for this change is that keeping alternative
names as a newline separated list in a single column doesn't
fit the relational model well, and makes it awkward to query
on.

This commit also replaces name-related methods with ones that
use alternative_names, and updates the tests that used to
relate to other_names.
Remove the now unused other_names column of Person
Update tests: "login" & "logout" are no longer on the Kenya homepage
Remove the <strong> around additional names

Since the <dt> styles also embolden, it looks strange.  At the
moment this wouldn't have been obvious since there wasn't more
than one name in other_names in the Kenyan Mzalendo.
Fix the old migration 0038_set_country_for_aspirant_presidents

In South datamigrations you have to refer to models with
orm.ParticularModel instead of just ParticularModel, so that
the frozen version of the model is used instead.  This wasn't
a problem when this migration was run, but may be in the
future if the models that it refers to change.
Fix the old migration 0037_migrate_data_for_parliamentary_sessions

South datamigrations have to use the frozen models via the orm
parameter in the forwards and backwards migrations.  When doing
the migration in an admin command, this wasn't the case - it
was importing whatever the models were in the current models.py.
Instead, remove the admin command and use the code in the
datamigration itself, referring to the models via the orm
object.