Fix CSP for disqus and Google Analytics

This commit should fix existing problems with Disqus and Google
Analytics enabled in the meta-yaml section of a note.

Before this commit they were blocked by the strict CSP. It's still
possible to disable the added directives using `addDisqus` and
`addGoogleAnalytics` in the `csp` config section.

They are enabled by default to prevent breaking changes.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

config.json.example

@@ -27,7 +27,9 @@
             "directives": {
             },
             "upgradeInsecureRequests": "auto"
-            "addDefaults": true
+            "addDefaults": true,
+            "addDisqus": true,
+            "addGoogleAnalytics": true
         },
         "db": {
             "username": "",