fix: don't trust libmagic charset recognition

[jennifer-richards]

Because libmagic does not examine a file's full contents, it sometimes mistakes utf-8 or other encodings for ascii. To avoid failures when it's wrong, treat ascii as utf-8 by default, and fall back to latin-1 (iso8859-1) if that fails.

Always encode rendered markdown as utf-8, falling back to its original charset is silly.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (4580a13) to head (639fa7b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/meeting/views.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9815      +/-   ##
==========================================
- Coverage   88.55%   88.54%   -0.01%     
==========================================
  Files         316      316              
  Lines       42256    42261       +5     
==========================================
+ Hits        37420    37422       +2     
- Misses       4836     4839       +3     

☔ 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.