Delete legacy data

This data was imported long ago and isn't required anymore.
Get rid of Travis config

Now that Jenkins does branch builds, we don't use Travis anymore.
Delete old migrations

Old migrations are not needed any more. This gets rid of all those older
than one month.
Delete old one-off scripts

These have all passed their sell by date and are no longer used. The
cost of maintaining/updating them when stuff changes far outweighs the
benefit of having them hang around just in case we need them again one
day.
Get rid of redundant seed data

None of this is used anymore.
Merge pull request #1560 from alphagov/delete-cruft

Delete cruft