Allow explicit loading of ActiveStorage routes

[justinko]

Fixes #31228

Motivation / Background

Users are creating a "catchall" route in routes.rb that will always precede the ActiveStorage routes. Railtie routes that are loaded after the app's routes are getting caught in it.

Detail

With this change, you can now load the routes when you want by doing the following:

Rails.application.config.active_storage.draw_routes = false

# in your routes.rb
Rails.application.routes.draw do
  ActiveStorage::Routes.draw(self)
end

Additional information

Should we do this for e.g. ActionMailbox as well?

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.