Include Gravatar

[Jskobos]

Gravatar will make a good default avatar image until we can implement uploading/storing/editing user avatars.

In order to make the request to the Gravatar API, we need the MD5 hash of the user's email address.
This is better done on the server side, so we can include a hash in the Graphql queries.

[ortonomy]

Added an issue to backend -- ortonomy/fling-server#9

[ortonomy]

This will need either md5 hashing of the email address or building a function inside schema to store MD5 hash of email and supplying it as part of thisUser. Might be a good first issue for someone to work on front-end and backend.