fix: more submission date feedback; refactor xml2rfc log capture

[jennifer-richards]

Main goal: log helpful feedback on more (hopefully now all!) paths where a <date> fails parsing or interpretation.

Throws in some refactoring to centralize capture of the xml2rfc log output. This is done with the capture_xml2rfc_output context manager. A known side effect of the refactoring in 3529f14 is that xml2rfc.log.write_out and xml2rfc.log.write_err will be restored to their original streams after each call to render_missing_formats() instead of being left pointing at otherwise abandoned StringIO instances. I think that's a win, but wanted to call it out as a possibly non-obvious change.

[codecov]

Codecov Report

Attention: Patch coverage is 85.93750% with 9 lines in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (70bf2ae) to head (e61d78a).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/submit/utils.py	78.94%	8 Missing ⚠️
ietf/utils/xmldraft.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8621      +/-   ##
==========================================
- Coverage   88.88%   88.80%   -0.08%     
==========================================
  Files         312      314       +2     
  Lines       40891    41221     +330     
==========================================
+ Hits        36345    36606     +261     
- Misses       4546     4615      +69     

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[kesara]

LGTM.