Add avatar's import from disqus

[umputun]

As mentioned in https://github.com/umputun/remark42/issues/958#issuecomment-828707386 Disqus has the API to retrieve avatars. Adding this to the import process will be very nice.

A few challenges:

• probably the API calls should be rate-limited and may need retries
• imported avatars should be proxied, i.e. stored and served locally

We already support avatar proxy as a part of auth, but this is an external library. Not sure if we can make the avatar proxy part available in go-pkgz/auth or just copy the relevant part into remark42