Moved explanation of `--mountable` option up

The example code uses `--mountable`. In previous version this was followed by an explanation of the `--full` option, which doesn't make sense. Later on in the text this is better explained. I moved this explanation up without repeating too much.
skip migration check if adapter doesn't support it
Prevent state leak.
Remove redundant code.
Move cleanup into teardown, delete obsolete code.
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]
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