pep8/flake8 code cleanup
when saving internal attendees, don't overwrite already existing attendance status. Fixes #9819
Refs #9819 Update the tests to include the default 'attending' field

See also: e403d3856ce32dea8be93fb689872ca5055aefad
moved subject to template to be able to customize it, refs #8077
new
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fixed mime type, refs #10107
Preparing release 4.0-P3-1
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decode utf-8 to avoid UnicodeDecodeError
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