Fix Sprockets 4 support for extensions

[aldesantis]

Description

This fixes an issue with extension tests failing due to Sprockets 4 requiring an app/assets/images directory to be present.

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)

[kennyadsl]

We'll need this PR to fix all extensions specs, but if #3379 will be merged only. Otherwise, we can close this PR and backport the Sprockets 3 lock to all the supported Solidus versions.

[aldesantis]

I'll close this in the meantime since we've locked Solidus to Sprockets 3. If rails/sprockets-rails#446 is merged, Solidus may become compatible with Sprockets 4, in which case I'll reopen it.

[aldesantis]

Reopening this, since:

• After discussing with @kennyadsl, we decided we really really want to support Sprockets 4, especially considering that someone may have upgraded their store to Sprockets 4 already.
• The fix in Allow using a custom assets configuration manifest rails/sprockets-rails#446 only solves Sprockets 4 issues in the core dummy app, while this one solves a different issue with the extensions dummy app.

Sorry about the confusion!

Note that this will also need to be backported to all currently supported Solidus version branches. We won't need to release new versions, since it's a dev-only change and extensions use branches rather than tags.

[jacobherrington]

I am happy with merging this, but I'd like to see a comment explaining why we need to do this or at least linking back to this PR.