Use Spree.user_class instead of Spree::LegacyUser in production code

[mamhoff]

Other apps will probably have different user classes, and need to use
them.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[mamhoff]

This probably wants to be backported to Solidus 2.11.

[kennyadsl]

Thanks, going to merge and backport to 2.11 without waiting for 24h in this case.

[spaghetticode]

@mamhoff thanks!