Allow to disable gravatar

Since Gravatar is an external image source and not perfect from a
privacy perspective, forbidding it allows to improve privacy.

This commit also simplifies and optimizes the avatar code.

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

lib/config/default.js

@@ -146,5 +146,6 @@ module.exports = {
   },
   email: true,
   allowEmailRegister: true,
+  allowGravatar: true,
   allowPDFExport: true
 }