Stop recommending deprecated CSP directive plugin-types in the content repository part of the spec

[dkasak]

The plugin-types directive was removed from CSP, both in the web spec and in Chrome/Chromium. Firefox never implemented this.

Previously, a valid use for it was to specify plugin-types: application/pdf since object-src: 'none' disallowed rendering PDFs in some browsers using the browser's built-in PDF renderer (notably this was the case for Chrome). This is also why we started doing it and recommending it. Supposedly another workaround for this was to add rel="noreferrer" (see: https://secure.phabricator.com/T13112#237527).

Since support for this was removed, hopefully this shouldn't be a problem anymore. We should test whether it's still happening. If not, we should just switch to using/recommending object-src: none instead. If it's still happening, we should investigate whether there's another workaround we can use.