Parsing DATABASE_URI, use URI#hostname: it's smarter about IPv6

Fixes #15705.
SecurePassword - Validate password must be less than or equal to 72
See #14591, Reason - BCrypt hash function can handle maximum 72 characters.
Merge pull request #15708 from akshay-vishnoi/secure_password

SecurePassword - Validate password must be less than or equal to 72
Added changelog for #15708 [ci skip]
Cleaned up the `has_secure_password` test cases

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