Profile settings: refresh avatar option.

When a user logs in for the first time via GitHub or Google, Sandstorm queries the identity provider for a profile picture and uses that. However, if the user later changes their profile picture via the identity provider, we never get the update, and the user has no way to propagate the update other than to manually upload the same image. We probably don't want to be polling continuously, but it would be nice if there was just a button to refresh it directly.

Note also that, if we added such a button, we could use a similar flow to let users import a picture from gravatar for email logins -- right now we don't query gravatar on account creation because that would be a privacy leak, but an "import from gravatar" button wouldn't be an issue, since the user is opting in to the fetch.