`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
Add docs for AS::TestCase::test_order

Document `test_order` and `test_order=` from `ActiveSupport::TestCase`.

[ci skip]
Merge pull request #18077 from claudiob/add-docs-for-test-order

Add docs for AS::TestCase::test_order
Enable Travis CI container-mode

And also bundler cache
Output extension is no longer generated [ci skip]

/cc @senny Is this change documented somewhere?
Revert "Merge pull request #18003 from sikachu/permit_all_parameters-thread-safety"

This reverts commit da5cc10e945552da54234f858470238a3fc36767.

Fixes #18091

See also https://github.com/rails/rails/pull/18003#commitcomment-9030909
Use released rack

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446
`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
Use released I18n 0.7.0
Remove rack and i18n master from bug report tempaltes [ci skip]