Deprecate `serialized_attributes` without replacement

Clean up I18n stored translations after test.

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

Closes #15721

/cc @rafaelfranca

[ci skip] Update #has_secure_password docs

We've stopped using it internally, in favor of polymorphism. So should
you!

Use `@existing_user` while updating existing user, fixing - ee4e86

Avoid hard-coded value in test setup and teardown.

Make sure restoration always happen.

* Grouped the valid test cases in one place
* Make the length of the generated password obvious
* Removed two wrong (copy-and-pasted) test cases

SecurePassword - Validate password must be less than or equal to 72

See #14591, Reason - BCrypt hash function can handle maximum 72 characters.

Fixes #15705.

Remove `serialized?` from the type interface

Fix request's path_info when a rack app mounted at '/'.

Open extension point for defining options in build_through_record

Remove unused `method_missing` definition

This fixes #15496

Add missing test cases for #assert_no_difference

prevent bad automatic inverse_of association

In actionview, eliminate calls to tag that use html_safe parameter values.

Fixes issue #15511.

Through associations should set both parent ids on join models

ActionController::Parameters#require now accepts FalseClass values

member = Member.new(club: Club.new)
member.save!

Before:

member.current_membership.club_id # => nil

After:

member.current_membership.club_id # => club's id

Detect in-place changes on mutable AR attributes

We have several mutable types on Active Record now. (Serialized, JSON,
HStore). We need to be able to detect if these have been modified in
place.

Remove unused column types override

We always define attribute methods in the constructor or in `init_with`.

Reorder test which does not represent real world usage

variables [ci skip]
Thanks to @matthewd for the excellent feedback! ❤