Migration to add 'the' to will_continue_on text

www.agileplannerapp.com/boards/173808/cards/8718

Frontend [will stop adding 'the'](https://github.com/alphagov/frontend/pull/695) to `will_continue_on` text on business support editions. This change allows editors to add 'the' when needed in the best way possible, and keeps it consistent with other formats.

This migration fixes existing data to contain that word. Also, updated the hint text to suggest that editors need to add 'the' to `will_continue_on` in business support.
Will this migration also add "the" to empty `will_continue_on` fields?