Refactor load_user_roles into current_user_roles helper

[mamhoff]

Summary

Prior to this commit, we had a before action populating the current_user_roles instance variable, and then we used that instance variable in helpers. This refactors it into an actual helper that we can then use as we need.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.82%. Comparing base (0ba0738) to head (58e255c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
api/app/controllers/spree/api/base_controller.rb	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6245      +/-   ##
==========================================
- Coverage   88.83%   88.82%   -0.01%     
==========================================
  Files         850      850              
  Lines       18329    18334       +5     
==========================================
+ Hits        16282    16286       +4     
- Misses       2047     2048       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.