Add ability to ignore avatar

[cxcorp]

We operate a IRC<->Telegram bridge, and have noticed that we get a lot of unnecessary file uploads from user avatars. We don't really need the avatar images at all, as we only bridge to IRC.

I think that it would be nice to have a configuration setting which prevents Matterbridge from uploading user avatars to the media server. This would have the benefit of reduced bandwidth and storage consumption, especially when media is uploaded to AWS S3 like in our case.

I tried to find a setting related to this but couldn't find one, so this is a feature request :)