JMAP Email: rewrite JMAP Email to MIME conversion

[rsto]

This updates how MIME messages are created from JMAP Email objects for Email/set{create}. Changes include:

• Inline plain text is rewritten before conversion regardless if its content is sourced from an EmailBodyValue or Blob. Non-plain text content now never is rewritten, even if it is defined with in a EmailBodyValue.
• If the charset property for inline plain text declares the content to be ASCII or UTF-8, then the EmailBodyPart is rejected for invalid characters.
• The content transfer encoding for "message" media types is chosen by the media subtype, e.g. handling "message/rfc822" and "message/global" differently.
• Bare carriage returns in inline plain text now get expanded to CRLF, rather than omitted from the body part contents.
• Transfer encoding names are written in lowercase, e.g. "8bit" rather than "8BIT", as recommended by

[rsto]

This is now part of #4926