`db:structure:load` and `db:schema:load` no longer purge the database.

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems
`force: :cascade` to recreate tables referenced by foreign-keys.
Merge pull request #18082 from rails/17945_purge_in_test_only

`db:schema:load` and `db:structure:load` do not purge the database