fix(sec): Overwrite ImageMagick's security policy to tighten it up (part1)

[antobinary]

The default policy.xml for ImageMagick needed to be sealed further for us to operate safely.

This is part of https://github.com/bigbluebutton/bigbluebutton/security/advisories/GHSA-v28f-mvg5-mv7m which will be published soon.

We recommend that administrators apply the changes to /etc/ImageMagick-6/policy.xml whether via bbb-install.sh or through other means.

Note: to improve usability, consider dropping SVG from the list of supported presentation file formats via:

Adding to /etc/bigbluebutton/bbb-html5.yml (and restart BBB afterwards)

public:
  presentation:
    uploadValidMimeTypes:
      - extension: .pdf
        mime: application/pdf
      - extension: .doc
        mime: application/msword
      - extension: .docx
        mime: application/vnd.openxmlformats-officedocument.wordprocessingml.document
      - extension: .xls
        mime: application/vnd.ms-excel
      - extension: .xlsx
        mime: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
      - extension: .ppt
        mime: application/vnd.ms-powerpoint
      - extension: .pptx
        mime: application/vnd.openxmlformats-officedocument.presentationml.presentation
      - extension: .txt
        mime: text/plain
      - extension: .rtf
        mime: application/rtf
      - extension: .odt
        mime: application/vnd.oasis.opendocument.text
      - extension: .ods
        mime: application/vnd.oasis.opendocument.spreadsheet
      - extension: .odp
        mime: application/vnd.oasis.opendocument.presentation
      - extension: .odg
        mime: application/vnd.oasis.opendocument.graphics
      - extension: .jpg
        mime: image/jpeg
      - extension: .jpeg
        mime: image/jpeg
      - extension: .png
        mime: image/png
      - extension: .webp
        mime: image/webp

[antobinary]

Part 2 #812 is mandatory!!