optionally blur NSFW content in lieu of placeholder image

This MR adds a new configuration setting nsfwUseBlur which uses a client-composited blur of the image or first video frame.

In the case of images, it also adds placeholder text that explains that the content is sensitive. For videos, the play icon is used.

In the event that attachment post-processing is available, such as when running on Mastodon BE or MFC Pleroma, the post-processed image will be used as the basis for the blur composition instead of the raw image or first video frame.