Blacklist pdf

See merge request !2

Add a setting to disable the plugin functionality

See merge request !1

The setting is default-enabled, which should allow network activation
for all blogs on a multisite instance while letting individual blogs
retake control in case of problems.

Video is skipped due to the (current) lack of ffmpeg in wp-mat-server.

Should avoid failure of the is_uploaded_file() check.

POC