Clean up I18n stored translations after test.
Deprecate `serialized_attributes` without replacement

We've stopped using it internally, in favor of polymorphism. So should
you!
[ci skip] Update #has_secure_password docs
Merge pull request #15724 from akshay-vishnoi/secure_password_docs

[ci skip] Update #has_secure_password docs
docs, remove getting started guide sample application. [ci skip]

This application always gets out of sync and doesn't add much value.

Closes #15721

/cc @rafaelfranca
Merge pull request #15715 from zuhao/refactor_actionview_cleanup_I18n_translations

Clean up I18n stored translations after test.
Merge pull request #15704 from sgrif/sg-deprecate-serialized

Deprecate `serialized_attributes` without replacement
[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
Use singular routes when plural routes do not exist

We have two distinct problems here. For new records, it's much simpler
-- The detected index route will not exist, so we can simply check
`respond_to`.

With show actions, the detected helper method will exist,
but take one fewer argument than we think. The method itself is
splatting args, so it's difficult to make a determination based on the
arity of the function. It seems the simplest thing to do here is check
if a model was passed as the format argument.