Add templates and sections to shop

[denny]

The code for this is currently very heavily based on the Page/Section/Template stack, so hopefully somewhat proven already.

The differences are mostly in how the deletion of products is handled (Stripe doesn't allow it), and hence some of the stuff around sections has to be different too.

[codecov]

Codecov Report

❌ Patch coverage is 98.46626% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.91%. Comparing base (fb9c90d) to head (13c7c4b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...inyShop/app/helpers/shiny_shop/main_site_helper.rb	66.66%	4 Missing ⚠️
plugins/ShinyShop/app/models/shiny_shop/section.rb	97.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #2075      +/-   ##
===========================================
- Coverage   100.00%   99.91%   -0.09%     
===========================================
  Files          418      428      +10     
  Lines         5864     6175     +311     
===========================================
+ Hits          5864     6170     +306     
- Misses           0        5       +5     

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