Limit avatar download attempts

[jcansdale]

This PR limits the number of attempts that will be made to download avatar when a host requires authentication (which isn't currently supported for images).

GitHub Enterprise returns a 200/OK with a content type of text/html when authentication is required to download an avatar.

What this PR does

• When an avatar returns a non-image content type, remember this for subsequent calls to the same host

Reviewers

• Thanks @darn for confirming the fix: Avatars being repeatedly downloaded. #1547 (comment)

Todo

• We should refine the type and/or number of exceptions that can be thrown before a host is blacklisted to make it more robust.
  • Changed to only remember exceptions caused by hosts returning non-image content

Fixes #1547

[grokys]

My only problem with this is that it will still be throwing an exception for each requested avatar. That could be avoided by putting the blacklisting into AvatarProvider. However given our schedule and that this is a verified solution to a relatively serious problem, I'm 👍.