P2: add a smooth CSS transition to gravatars on focus

[audrasjb]

Here is a simple patch to add a simple CSS transition between normal and focused states when you put the focus/hover on Make P2's gravatars (for both post authors and commenters).

The screenshot below is using this CSS rule:

.author-avatar img {
    transition: all .5s;
}

See screenshots below.

[audrasjb]

Current behavior: no transition between rounded and squared gravatar image.

[audrasjb]

Proposal: a 0.5s transition to make it smoothier :-)

[dd32]

In 9623:

Make: Hide the gravatar-hover preview images that have a mismatched size.

This also removes a circle -> square upon hover that seems irrelevant.

Fixes #4635, #4918.