Twenty Thirteen: Move no-avatar class to the body

[obenland]

We can simplify the way we handle the event of disabled avatars.
Let's move it to the 'body_class', this way we only have to output it once, don't have to filter every comment, and can cut down on code in functions.php.

[nacin]

In 24031:

Twenty Thirteen: Move no-avatars class to the existing body class modification function.

• Move includes to the top of the file.
• Make search_form two words in twentythirteen_search_form_format().

fixes #24127.

[obenland]

For nacin's consideration.