Merge pull request #15710 from zuhao/refactor_actionview_digestor_test

Make sure restoration always happen.
Avoid hard-coded value in test setup and teardown.
Clean up I18n stored translations after test.
Add test cases for Hash#asset_valid_keys
Merge pull request #15712 from zuhao/refactor_actionview_javascript_helper_test

Avoid hard-coded value in test setup and teardown.
Use `@existing_user` while updating existing user, fixing - #ee4e86
pg guide, use the term `database views` to be specific. [ci skip]
Fixed a regression introduced in 84cf156

84cf156 (PR #15694) introduced a subtle regression. There are actually three
distinct entry points to creating an AR object – via .new (i.e. #initialize),
via #init_with (e.g. from YAML or database queries) and via .allocate.

With the patch in 84cf156, attribute methods and respond_to? will not work
correctly when objects are allocated directly, without going through either

The reason this test case didn't catch the regression was that the `Topic`
class is shared between test cases, so by the time this test case is ran the
attribute methods are very likely to be defined.

Switching to use a fresh anonymous class in the test to ensure we surface this
problem in the future.
Merge pull request #15717 from akshay-vishnoi/test-cases

Use `@existing_user` while updating existing user, fixing - ee4e86
remove unnecessary substitution for space in ActionView::Helpers#debug
Add an option to skip installation of Turbolinks
/mysql/i -> MySQL, Spell correct in continuation to #15555
[ci skip] Use `an` for SQL
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