Split middlenames out of actor table

[eFuller96]

The actor table contains a first_name column which actually contains first names and middle names.

This PR adds a new migration to split the first name column into two: first_name and middle_name. Where actors have middle names, their middle name will be moved to the middle_name column and first_name will have the middle name removed.

Actors without middle names should be unaffected.

This PR also adds a test for this migration to ensure that we capture the intended behaviour of the migration script. We dump the state of the actors table before the migration, then dump the state of the actors table after the migration and run a diff against the two (after rejoining the names appropriately) to ensure that the migration completed as intended.