Use ActionView for emails

[Hamms]

I ran lib/scripts/test_poste_deliveries.rb (added in #33720) successfully on production and it did not generate any HoneyBadger errors, so we can confidently switch over to the ActionView implementation of email sending (added in #33399) from TextRender.

The template changes here look more significant than they are; currently, we have an .actionview template for each regular template: https://github.com/code-dot-org/code-dot-org/tree/92ad7c3/pegasus/emails. This change just replaces the regular template with the actionview version, so we have just the one: https://github.com/code-dot-org/code-dot-org/tree/2fc3db9/pegasus/emails

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[islemaster]

🎉 This is so awesome! Thanks for the thorough test here.