Allow modify CSP list for embeeded images

[nickvergessen]

Feedback from #277 by @splitt3r

External mages won´t work by intention right? Maybe a small hint about it? Could be very interesting vor announcements. Sth. like an marketing image from the companies website.

Steps to reproduce

1. Post an image markdown e.g. from github: https://raw.githubusercontent.com/nextcloud/announcementcenter/master/docs/AnnouncementCenterFrontpage.png

Expected behaviour

🖼️ Works for some domains

Actual behaviour

🛑 CSP error