Add Option to use Gravatar as Fallback for Profile Image

[santonelli]

Summary

We can add an option to use Gravatar as fallback to display a user's profile image. If enabled and if there is no image set, we can try to fetch an image.

Acceptance Criteria

• It's done when there is an option in @@sidebar-settings to enable this feature
• It's done when we fetch an image from Gravatar if no profile image is set