Create a custom log formatter that allows log entries to contain wikitext

Create a custom log formatter that allows log entries to contain wikitext

This creates a new log formatter called WikitextLogFormatter, which
is a simple class that allows log entries to contain properly
formatted wikitext. This makes it easier for extensions so they don't
have to create their own subclass of LogFormatter.

Change-Id: I2b7fddf5c6ef017a0925b4bf75cfd47cb55aa5de