PDF presentations

This pull-request adds support for PDF presentations:
- Enable through the header bar when in a call / conference
- Drop PDF file into the presentation area
- Peers will be notified about new presentation and download it
- Anybody can navigate through the presentation pages which will be synchronized between peers
- Pages will scale to available space

Make sure to rebuild styles after merging!

Open issues:
- update release target to include pdf.js worker file
- only load pdf.js if presentations are used and don't include in main javascript files