Fix `ActiveRecord::RecordNotFound` error message with custom primary key
Merge pull request #13785 from kuldeepaggarwal/fix-find_with_multiple_ids

Fix `ActiveRecord::RecordNotFound` error message with custom primary key
Remove unused argument.
Rewrote the tests for has_secure_password
Restored the ability to clear the password with user.password= nil (see the docs)
Got all the new tests passing
Removed old tests
Some minor fixes
Merge pull request #13835 from frodsan/remove_unused_argument

Remove unused argument.
Merge pull request #13772 from chancancode/fix_has_secure_password_for_good

Refactored validations rules for has_secure_password
Transform dashes to underscores in resource route names

Fixes #13824
Add additional tests for #13824
Add a missing changelog entry for #13825 [ci skip]
Merge pull request #13841 from robin850/patch-15

Add a missing changelog entry for #13825 [ci skip]
Correct grammar from '... allowing both thread web servers ...' to '... allowing both threaded web servers ...'.
Merge pull request #13844 from Alamoz/config_grammar

thread -> threaded [ci skip]
Add the ability to intercept emails before previewing

To support the ability for tools like CSS style inliners to operate on emails
being previewed this commit adds a hook in a similar fashion to the existing
delivery interceptor hook, e.g:

  class CSSInlineStyler
    def self.previewing_email(message)
      # inline CSS styles
    end
  end

  ActionMailer::Base.register_preview_interceptor CSSInlineStyler

Fixes #13622.
Don't use a class_attribute for ActionMailer::Base.preview_path

Since preview_path is read from ActionMailer::Base when previewing, subclasses
can’t change it so don’t there's no need for the extra overhead imposed by using it.
Support underscored symbols in Action Mailer config

We allow the use of underscored symbols to represent classes throughout
other parts of Rails so it seems incongruous that it's not supported in
`register_interceptor` and `register_observer`.
Remove an extra comment [ci skip]