Update migrations

Resetting the column information of the models in the system means that I can
load the seed data immediately after running the migrations.
Add seed data

I want to populate the demo site with dummy data so that people can get an
idea of how it might work.
Disable uploads in 'demo mode'

Now that I've got some seed data I can use to demonstrate money tracker, I'd
rather avoid the chance of someone uploading their personal data.