remove or disable Gravatar by default and add warning text

[andreasnrb]

Since Gravatars can be used to identify individuals that want to be anonymous Gravatar functionality should either be removed or disabled by default. If it remains there should be a warning text to the site owner that their commentators might be identified using it. Also optin when commenting for use with gravatar should be required by default.

We cannot assume that people are aware that Gravatars can be used to identify them using their emails. This feature remaining on by default and in core essentially says that WordPress core developers don't care about their users privacy.
This is has been done and will continue to be done in the future. Latest case was with Disqus and them using Gravatar. But there is no problem with identifying people commenting on WordPress sites.

[SergeyBiryukov]

Duplicate of #14682.