Convert HTML metadatas as markdown

1. Remove MetaType.HTML , and introduce MetaType.Markdown instead
2. Store the raw markdown in database
3. Use https://python-markdown.github.io/ to render the input when giving the output to the API
4. Write some unit tests with valid markdown, and some html that should not be rendered

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cli.request('CreateMetaData', id=ELEMENT_ID, body={
    "type": "html",
    "name": SOME_NAME,
    "value": '<style type="text/css">* { display: none !important; }</style>',
})

Dès qu'on affichera l'élément, tout disparaît. On peut injecter n'importe quel HTML, CSS, JS, sans droits d'administrateur sur n'importe quel corpus accessible en écriture.

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Imported from Trello