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 89.65517% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.73%. Comparing base (f380b1a) to head (0fbc8f8).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
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   88.74%   88.73%   -0.02%     
==========================================
  Files         321      320       -1     
  Lines       41853    41656     -197     
==========================================
- Hits        37144    36962     -182     
+ Misses       4709     4694      -15     

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