Adding mediaSrc to CSP so video and audio files can be embedded without
problems.
From a security perspective it should be fine to load audio and video
data without introducing a high security issue. Only from a privacy
perspective it allows another way to track users if there are data
embedded. But it doesn't introduce any new attack vector as pictures are
also allowed from everywhere.
Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>