Sanitize CSS by removing URLs and @import / @charset directives.

Set some WP SuperCache CDN options site-wide (to hard-coded constants
-- this is improvable).

See issue noblogs-composer#24 for further details.

I think this should close #21. On the other hand, it says "this video is not
available in your region", so I am not sure this actually works.

Adds filters on the MIME type checks when uploading files.

Currently it allows to upload text files containing GPG/PGP keys.
It closes issue noblogs-composer#30

In the future we might need to add other filters.

see noblogs-composer#26

PHP prefers/requires function names to be quoted when passed as arguments

This reverts commit 483610dd