Move subclasses of LogFormatter to their own file

Except LegacyLogFormatter.

includes/logging/LogFormatter.php is getting bigger and bigger with
the addition of new formatters when logs are changed to use them
instead of the old method, so better doing this now before we get
new formatters for remaining log types.

Change-Id: I6aab19c8d68bf47beddad42632b0360a7b12f251
Merge "Move subclasses of LogFormatter to their own file"