[avatar-upload] Base for the new avatar feature 

This changes include the avatar upload itself, changes to the form and handling the image upload.

TODO:

[ ] Create an Image wrapper method and define a route to output the image
[ ] Resize the image and possibly crop after upload
[ ] Create a Twig function to handle the avatars - check if the user has an uploaded avatar. if not, try to use gravatar
[ ] Change all front-end rendering of avatars to use this Twig function