schema: extend content accepted HTML tags & attrs

[Samk13]

❤️ Thank you for your contribution!

Description

This PR introduces two new configurations in the Invenio pages module:
PAGES_ALLOWED_EXTRA_HTML_ATTRS and PAGES_ALLOWED_EXTRA_HTML_TAGS
These allow instance-level customization of accepted HTML tags and attributes, enhancing content flexibility.

By default, it includes <img /> and <button /> elements.
The extended whitelist of attributes is as follows:

<img />: ["src", "alt", "title", "width", "height"]
<button />: ["type", "name", "value", "disabled", "onclick"]

Checklist

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[ntarocco]

This looks very nice, thanks a lot! I added a comment to make it even more extensible.

[ntarocco]

Looks very good, I have added a small last suggestion ☺️

[ntarocco]

Thanks a lot for the extensive testing!