Feature Request: Ability to add additional variables to CSP headers

This would be very useful for admins to be able to make customisations like embedding a PeerTube video on their about page or something like an off-site uptime tracker in their sidebar.

It can also cause issues with attachments if you still have some legacy stuff served on a different domain than you do now.

Right now the only workaround is for admins to disable CSP in Pleroma and implement them in Nginx themselves.

However, because CSP headers aren't really documented and changes to them aren't communicated in the changelog it can be unwieldy and cause security issues.

Ideally, this is something that could be set in the config file and admin-fe.