Generate mailto: reply link with prefilled quiz body in quiz emails

[payamnj]

Part of #727.

Summary

Add a mailto: reply link to the quiz email — for courses whose Course.imap_connection is set — with a URL-encoded, prefilled subject and body so a learner can answer and submit a quiz by replying to the email, without needing an AMP-capable client.

Where this plugs in

• SendQuizCommand (django_email_learning/services/command_models/send_quiz_command.py) builds the render context (quiz, link, question_ids, token, etc.) and renders emails/quiz.txt, emails/quiz.html, and (if AMP-enabled) emails/quiz_amp.html.
• Add a new context value, e.g. mailto_link, computed only when content.course.imap_connection_id is set.
• Render into both emails/quiz.html and emails/quiz.txt (both are non-interactive fallbacks today — a plain "click here" link).

Behavior

• Subject: quiz <token> — same token value already used to build link (see DeliverySchedule.generate_link(), django_email_learning/models/deliveries.py).
• Body: for each question in quiz.questions filtered to question_ids (same filtering emails/quiz_amp.html already does so the mailto body matches exactly what an AMP-capable client would show), render the question text and its answers as [ ] checkbox lines. Use question.is_multiple_choice() to phrase the instruction per question ("select the correct answer" vs. "select all correct answers"). End with a short instruction to mark [x] and reply/send.
• URL-encode subject and body appropriately for a mailto: href (line breaks, special characters).
• Keep the existing web link as-is in both templates — the mailto link is additive, not a replacement (some clients don't support mailto: body prefill well either, and the web link is the universal fallback).

Out of scope (see #727 for the other half)

• The IMAP-side parsing/handling of the reply itself.

Tests

• Extend tests/services/command_models/test_send_quiz_command.py: assert the mailto link/subject/body appear in the rendered email when the course has an imap_connection, and are absent when it doesn't.
• Cover a question with is_multiple_choice() == True and one with False to confirm the phrasing differs correctly.