fix: set charset for text/plain responses

[jennifer-richards]

The first commit fixes an observed bug with the iesg agenda_package.txt view, along with similar iesg views, by setting charset for the content type.

The second commit adjusts every place where we return an HTTP response with content_type="text/plain" to set the charset as well. Most of these could be left alone - the default is US-ASCII, and most of them are simple fixed US-ASCII strings - but it seemed reasonable to me to speak utf-8 everywhere.

Django will apply the encoding specified in the content_type to strings when it builds its response, so unless we're handling responses that include non-UTF-8 bytes or strings encoding characters UTF-8 can't represent, this should be safe. I didn't notice any of those, but it'd be good to check that.

Fixes #8968

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.72%. Comparing base (e16d568) to head (9c21501).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/api/views.py	87.50%	1 Missing ⚠️
ietf/doc/views_ballot.py	50.00%	1 Missing ⚠️
ietf/meeting/views.py	93.75%	1 Missing ⚠️
ietf/submit/views.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9120      +/-   ##
==========================================
- Coverage   88.72%   88.72%   -0.01%     
==========================================
  Files         321      321              
  Lines       41851    41851              
==========================================
- Hits        37134    37132       -2     
- Misses       4717     4719       +2     

☔ 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.