Resize avatars on upload

[crysfel]

We need to resize images to the size we need for avatars. For that we are going to use https://github.com/lovell/sharp

AC

• Install sharp for resizing images
• Create a thumbnail image from the original
• Set the avatar field in user record to the thumbnail image
• Make sure to remove avatars when removing a user
• Make sure to remove thumbnail when updating a new image

[moshfeu]

@crysfel , what's the status of this one? It blocks up from deploy this: https://github.com/Coding-Coach/find-a-mentor/pull/675 (I merged it but reverted. I will be happy to deploy it again)