Implement generic oembed support

[Anonyymi]

• Allow oembed content via embed form field
• Validate field input on backend to allow only the configured embeddable websites (eg. youtube)
• Map to database fields like so:
  • file = embed HTML
  • file_hex = embed type (eg. youtube, etc)
  • file_original = embed title
  • file_size = 0
  • file_size_orginal = ''
• Fetch and store thumbnail
• If both upload file and embed are provided, upload file is preferred and embed url discarded

oEmbed spec: https://oembed.com/

[Anonyymi]

Done.