Add data: URL to CSP and upgrade helmet

Seems like the old version of helmet had a problem with `data:`. This
patch upgrades to the latest version and adds the CSP rule to allow
Google Fonts and the offline version of it, to properly include the
fonts and no longer throw ugly error messages at us.

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

package.json

@@ -46,7 +46,7 @@
     "gist-embed": "~2.6.0",
     "graceful-fs": "^4.1.11",
     "handlebars": "^4.0.6",
-    "helmet": "^3.3.0",
+    "helmet": "^3.13.0",
     "highlight.js": "~9.12.0",
     "i18n": "^0.8.3",
     "imgur": "git+https://github.com/hackmdio/node-imgur.git",