Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score
Removing unnecessary File.join calls
Reword documentation for update_all

It now contains a carefully formulated reference to the "current relation" which might help clarify that the receiving will generate its own scope, escaping the need for explicitly referencing `default_scope` which is, after all, just another way of specifying a scope and nothing special.
Document `String#html_safe` [ci skip]

It should be part of the documented public API, since we have an entire
section of the guides dedicated to it. Documented in a way that
addresses the concerns which kept it undocumented in the past.
Fix sprockets-rails dependency dance

- Remove sprockets-rails from generated Gemfile as rails has a hard-dependency on it

- Also allow sprockets-rails >= 2.0.0
Merge pull request #17752 from arthurnn/fix_sprockets-rails-dependency

Use sprockets-rails 2.2.x on base app and allow greater version
Merge pull request #17682 from ReneB/docs/update_all

Explain that default_scope also influences update_all
allow the "USING" statement to be specified on change column calls
oops, forgot to add the real assertion!
allow types to be passed in for USING casts

This allows us so abstract the migration from the type that is actually
used by Rails.  For example, ":string" may be a varchar or something,
but the framework does that translation, and the app shouldn't need to
know.
let mailer templates generate URLs by default [Xavier Noria, Richard Schneeman]
Merge pull request #17646 from andyjeffries/html_layout_fix

Creating mailer layouts by default, including html and body tags
Add tests which were incorrectly removed.
MailerGenerator should inherit from Base.
Generate mailer layouts even if no action is given.
Add missing CHANGELOG entry.
Update docs to reflect changes to MailerGenerator.
Merge pull request #17758 from tgxworld/mailer_generator

Follow up to #17646.
Deprecate string options in URL helpers

Fixes https://github.com/rails/rails/issues/16958

[Byron Bischoff & Melanie Gilman]