Add Gravatar url generation

[swilgosz]

Overview

Currently, we only have avatars for users when they register through Github. If someone registers using the standard login/password flow, we don't provide any defaults at the moment by the API.

Let's generate the gravatar using gem 'gravtastic'.

• The Avatar URL should be taken from the Github, if registration happened through OAuth flow
• Otherwise, the Gravatar Fallback Image should be returned.
• If no gravatar is set, let's return the placeholder image.