Susimail: Add support for HTML email
- HTML email shown in iframe with strict CSP
- Add CSPDetector to check user-agent for CSP support
- Update main CSP
- Use js from MR !160 (merged) to size iframe
- Use RegexOutputStream previously checked in to inject js to iframe
- Change RegexOutputStream to be case-insensitive
- Add new methods to fetch attachments by content ID
- Use RegexOutputStream to fixup content ID links
- Add support for multipart/related and prefer it
- Add links for showing alternate mail part (plain or HTML)
- Add Permissions-Policy header
- Add new configs allowHTML and preferHTML, default true