use HTML announcement email

[Roland Haas]

The current announcement email text is a plain text email with (most) links present in the mardown source removed.

It has been suggested to instead use a HTML email.

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[Roland Haas]

Any decision reached here should be recorded in the release process notes: https://docs.einsteintoolkit.org/et-docs/Release_Process#The_release

• pure ASCII with no line-breaks is required for hyperspace
• mailing lists archive only displays plain text part: http://lists.einsteintoolkit.org/mailman/private/maintainers/2020-May/001922.html (not publicly visible)

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