Mov/mp4 support

[apexskier]

Now that GitHub supports videos in comments, it appears they also support embedding certain videos within markdown files, for example: https://github.com/apexskier/rrf/blob/198908d/README.md.

However, if I want to host my video elsewhere to avoid adding a large file to my git repo, or for whatever other reason, camo breaks this because the file isn't an image. The link generated returns a Non-Image content-type returned error.

I don't know for sure, but I'd imagine this could be supported by adding video/quicktime and video/mp4 to the supported mime types and potentially changing the Content-Security-Policy header to use media-src in addition to/instead of img-src for video files.