[ci-skip] correct wikipedia MVC link
Edit pass on `has_secure_password` documentation [ci skip]
Merge pull request #15734 from aditya-kapoor/correct-link

[ci-skip] correct wikipedia MVC link
[ci skip] update wiki link for MVC
Merge pull request #15735 from aditya-kapoor/update-wiki-md

[ci skip] update wiki link for MVC in README
reload I18n as we are defining `:ts` locale
fix build- see https://travis-ci.org/rails/rails/jobs/27578159#L2913.
Merge pull request #15737 from kuldeepaggarwal/fix-build

reload I18n as we are defining `:ts` locale  in  `test_number_to_human_with_custom_translation_scope`
[ci skip] correct doc for `register_preview_interceptor`
remove unnecessary calling of `I18n.backend.store_translations(empty, {})`

Clean up I18n stored translations after test
continuation with #15714
Merge pull request #15738 from kuldeepaggarwal/remove-unwanted-code

remove unnecessary calling of `I18n.backend.store_translations('empty', {})`
Merge pull request #15739 from y-yagi/correct_doc_for_action_mailer_base

[ci skip] correct doc for `register_preview_interceptor`
allow preview interceptors to be registered through `config.action_mailer`.

This was partially broken because `preview_interceptors=` just assigned the
raw values, whithout going through `register_preview_interceptor`. Now the
Action Mailer railtie takes care of the `preview_interceptors` option.

This commit is a partial revert of:

Revert "Merge pull request #15739 from y-yagi/correct_doc_for_action_mailer_base"

This reverts commit a15704d7f35f17d34d0118546799141d6f853656, reversing
changes made to 1bd12a8609d275ad75fcc4b622ca4f5b32dc76be.

/cc @kuldeepaggarwal @y-yagi
Don't save through records twice

If the through record gets created in an `after_create` hook that is
defined before the association is defined (therefore after its
`after_create` hook) get saved twice. This ensures that the through
records are created only once, regardless of the order of the hooks.