Introduce Solidus update process

[waiting-for-dev]

Following the work done in #4064, this commits introduces a Rails
generator that creates a new initializer called, by default,
new_solidus_defaults.rb.

This initializer works in a very similar way that
new_framework_defaults.rb does in
Rails.
It allows users to preview the defaults that have changed on a new
Solidus version, as they are printed one by one on a commented line.
Users can then keep enabling them while updating their application code.

We're adding the load_defaults call with the old version value to the
generated initializer. Users can remove the file altogether when they're
done with the process. At that point, we require them to add
load_defaults with the new version to the main initializer file
(spree.rb). Even if there's no actual need for that, as
loaded_defaults on the configuration class defaults to the current
Solidus version, we want to enforce that so that users are on the safe
side for the next version upgrade. If they don't add it, we emit a
warning.

For now, we're leaving this as a generator, but we could reference it
from a rake task, although probably there's no need.

Be aware that users need to provide the version from which they are
updating. It's an option that offers more flexibility, as users can
update from versions different from the latest one. It also plays well
with our system's flexibility, for instance, to change defaults between
a pre-release and a release. However, we can add some code to default to
the latest minor version, but we should keep that information in our
code, and that's a small burden for our update process.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[waiting-for-dev]

@kennyadsl in the last commit, I added the warning prompted to the user that we discussed offline yesterday. We still need to keep track of the previous solidus minor version, so once it's merged, we need to update https://github.com/solidusio/solidus/wiki/How-to-release-Solidus

[kennyadsl]

Love it! Left some questions, though.

One extra step may be having a generic load_default function for users that has all the solidus components in place. Maybe we can automatically skip defaults for subcomponents (constants, like Spree::Backend) that are just not defined. Of course, if you think it's a good idea we can iterate on another PR. Thoughts?

Thanks!

[waiting-for-dev]

update.mp4

This is a short demo showcasing how the process works