Add script to change party memberships from ACN, ANPP and CPC to APC.

This is currently a very hard-coded script for Nigeria, but could be
worked on and moved later to make something more general.
End old party memberships the day before the new memberships start.
Change the party of exactly those who were members on the change date.

As it happens, this is everyone in the database, there is no-one in any
of these parties with a start or end date...
Merge branch 'issues/1105-change-party-memberships'