Fix: Favor HTML content instead of plain text

[Hugoch]

Hi,

On some email clients, plaintext attachment is shown instead of HTML (eg. Gmail). It is related to the correct interpretation of RFC 2046 5.4:

As with "multipart/mixed", the order of body parts is
significant. In this case, the alternatives appear in an order of
increasing faithfulness to the original content. In general, the
best choice is the LAST part of a type supported by the recipient
system's local environment.

To get HTML templates rendered as the default, we must attach plain-text message first, then HTML.
This PR updates the attachment order to get HTML first.

[neilmunday]

Thank you for this pull request @Hugoch and for the explanation - it will be included in the 4.12 release.

[github-actions]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/slurmmail
cli.py
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