Feature request: Add accessability alt tag data for avatar

[ipokkel]

Is your feature request related to a problem? Please describe.
Sometimes there is no Alt Text on avatar images

Describe the solution you'd like
A default alt tag, e.g. "user avatar", if no alt tag data exists for the avatar image.

Describe alternatives you've considered
Set a tag when none exists with custom code by hooking into basic_user_avatar_data.

[ipokkel]

Example of setting a custom alt tag when hooking into basic_user_avatar_data.

https://gist.github.com/ipokkel/4f5db2ec2b039dbb2ccdb6d9afe2e2a7