Fixes for r96546 (bug 33167):

Fixes for r96546 (bug 33167):
* Made LegacyLogFormatter try to respect the 'plaintext' parameter.
* Changed LogPage::addEntry() to use LogFormatter to get the action text as such calls to LogPage::actionText() are broken.
* Added comment to LogPage::actionText(). It's only OK to call this from the legacy log formatter, fetched via the proper factory functions.
* Converted CheckUser to using LogFormatter so that cuc_actiontext values work. Eventually, cuc_actiontext should be replaced with new cuc_params, cuc_log_type, and cuc_log_action fields, similar to the RC table.