revamp certificate and invite generation

[meaniebeanie22]

• [x] weasyprint rest api integration rather than xhtml
• [ ] change templates for weasyprint
• [ ] find nice certificate html templates and use the weasyprint rest api to pdfify them [let admins upload SANITISED html for their templates, with set data in tags]
• [ ] or use a pdf template and fill a field
• [ ] maybe talk to david and see how they did their certificates